diff --git a/domain-model.png b/domain-model.png
new file mode 100644
index 000000000..5e44d85ae
Binary files /dev/null and b/domain-model.png differ
diff --git a/src/main/java/com/booleanuk/core/TodoList.java b/src/main/java/com/booleanuk/core/TodoList.java
index 675adaf0b..17739e973 100644
--- a/src/main/java/com/booleanuk/core/TodoList.java
+++ b/src/main/java/com/booleanuk/core/TodoList.java
@@ -1,5 +1,108 @@
 package com.booleanuk.core;
 
+import java.util.*;
+
 public class TodoList {
 
+    Map<String, Boolean> tasks;
+
+    public TodoList(Map<String, Boolean> tasks) {
+        this.tasks = tasks;
+    }
+
+    public Boolean addTask(String taskName) {
+        taskName = taskName.strip().toLowerCase();
+        if (taskName.isBlank()) {
+            return null;
+        }
+
+        for (String key : tasks.keySet()) {
+            if (taskName.equals(key.toLowerCase())) {
+                return false;
+            }
+        }
+        tasks.put(taskName, false); // not completed by default
+        return true;
+    }
+
+    public List<String> getTasks() {
+        return new ArrayList<>(tasks.keySet());
+    }
+
+    public List<String> getCompletedTasks() {
+        List<String> completedTasks = new ArrayList<>();
+        for (String key : tasks.keySet()) {
+            if (tasks.get(key)) {
+                completedTasks.add(key);
+            }
+        }
+        return completedTasks;
+    }
+
+    public List<String> getIncompleteTasks() {
+        List<String> incompleteTasks = new ArrayList<>();
+        for (String key : tasks.keySet()) {
+            if (!tasks.get(key)) {
+                incompleteTasks.add(key);
+            }
+        }
+        return incompleteTasks;
+    }
+
+    public Boolean changeTaskStatus(String taskName) {
+        taskName = taskName.strip().toLowerCase();
+        if (taskName.isBlank())
+            return false;
+
+        for (String key : tasks.keySet()) {
+            if (taskName.equals(key.toLowerCase())) {
+                tasks.put(key, !tasks.get(key));
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public List<String> searchTask(String searchString) {
+        List<String> searchResults = new ArrayList<>();
+
+        for (String key : tasks.keySet()) {
+            if (key.toLowerCase().contains(searchString)) {
+                searchResults.add(key);
+            }
+        }
+
+        if (searchResults.isEmpty()) {
+            searchResults.add("No tasks found");
+        }
+
+        return searchResults;
+    }
+
+    public Boolean removeTask(String taskName) {
+        taskName = taskName.strip().toLowerCase();
+        if (taskName.isBlank()) {
+            return false;
+        }
+
+        for (String key : tasks.keySet()) {
+            if (taskName.equals(key.toLowerCase())) {
+                tasks.remove(key);
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public List<String> getAllTasksOrdered(boolean descending) {
+
+        List<String> sortedTasks = new ArrayList<>(tasks.keySet());
+
+        if (!descending) {
+            return sortedTasks.stream().sorted().toList();
+        } else {
+            return sortedTasks.stream().sorted().toList().reversed();
+        }
+    }
+
 }
diff --git a/src/main/java/com/booleanuk/extension/Task.java b/src/main/java/com/booleanuk/extension/Task.java
new file mode 100644
index 000000000..dc785981c
--- /dev/null
+++ b/src/main/java/com/booleanuk/extension/Task.java
@@ -0,0 +1,49 @@
+package com.booleanuk.extension;
+
+import java.time.LocalDateTime;
+
+public class Task {
+    private int id;
+    private String name;
+    private boolean completed;
+    private LocalDateTime creationDate;
+
+    public Task(int id, String name, boolean completed, LocalDateTime creationDate) {
+        this.id = id;
+        this.name = name;
+        this.completed = completed;
+        this.creationDate = creationDate;
+    }
+
+    public int getId() {
+        return id;
+    }
+
+    public void setId(int id) {
+        this.id = id;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public boolean isCompleted() {
+        return completed;
+    }
+
+    public void setCompleted(boolean completionStatus) {
+        this.completed = completionStatus;
+    }
+
+    public LocalDateTime getCreationDate() {
+        return creationDate;
+    }
+
+    public void setCreationDate(LocalDateTime creationDate) {
+        this.creationDate = creationDate;
+    }
+}
diff --git a/src/main/java/com/booleanuk/extension/TodoListExtension.java b/src/main/java/com/booleanuk/extension/TodoListExtension.java
new file mode 100644
index 000000000..d0c3c0e8e
--- /dev/null
+++ b/src/main/java/com/booleanuk/extension/TodoListExtension.java
@@ -0,0 +1,46 @@
+package com.booleanuk.extension;
+
+import java.time.LocalDateTime;
+import java.util.List;
+
+public class TodoListExtension {
+
+    List<Task> tasks;
+
+    public TodoListExtension(List<Task> tasks) {
+        this.tasks = tasks;
+    }
+
+    public String searchTaskById(int taskId) {
+        return tasks.stream()
+                .filter(t -> t.getId() == taskId)
+                .findFirst()
+                .map(Task::getName)
+                .orElse("No task found");
+    }
+
+    public Boolean updateTaskNameById(int taskId, String newName) {
+        return tasks.stream()
+                .filter(t -> t.getId() == taskId)
+                .findFirst()
+                .map(t -> { t.setName(newName); return true;})
+                .orElse(false);
+    }
+
+    public Boolean changeTaskStatusById(int taskId) {
+        return tasks.stream()
+                .filter(t -> t.getId() == taskId)
+                .findFirst()
+                .map(t -> {t.setCompleted(!t.isCompleted()); return true;})
+                .orElse(false);
+    }
+
+    public LocalDateTime getTaskCreationDate(int taskId) {
+        return tasks.stream()
+                .filter(t -> t.getId() == taskId)
+                .findFirst()
+                .map(Task::getCreationDate)
+                .orElse(null);
+    }
+
+}
diff --git a/src/test/java/com/booleanuk/core/TodoListTest.java b/src/test/java/com/booleanuk/core/TodoListTest.java
index 0bef779a4..fcb6c9e90 100644
--- a/src/test/java/com/booleanuk/core/TodoListTest.java
+++ b/src/test/java/com/booleanuk/core/TodoListTest.java
@@ -3,11 +3,262 @@
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+
 class TodoListTest {
+
+    Map<String, Boolean> tasks = null;
+
+    /*
+    addTask(String taskName)
+     */
+    @Test
+    public void addTaskNotAlreadyInTaskListShouldReturnTrue() {
+        this.tasks = new LinkedHashMap<>();
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(true, tl.addTask("Eat"));
+        Assertions.assertEquals(true, tl.addTask("Work out"));
+        Assertions.assertEquals(true, tl.addTask("Do some programming"));
+    }
+
+    @Test
+    public void addTaskAlreadyInTaskListShouldReturnFalse() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+        tasks.put("Work out", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(false, tl.addTask("Eat"));
+        Assertions.assertEquals(false, tl.addTask("eat"));
+        Assertions.assertEquals(false, tl.addTask("Work out"));
+        Assertions.assertEquals(false, tl.addTask("work out"));
+    }
+
+    @Test
+    public void addTaskInvalidNameShouldReturnNull() {
+        this.tasks = new LinkedHashMap<>();
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertNull(tl.addTask(" "));
+        Assertions.assertNull(tl.addTask("    "));
+        Assertions.assertNull(tl.addTask(""));
+    }
+
+    /*
+    getTasks()
+     */
+    @Test
+    public void getTasksTaskMapNotEmpty() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+        tasks.put("Work out", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of("Eat", "Work out"), tl.getTasks());
+    }
+
+    @Test
+    public void getTasksWhenMapIsEmpty() {
+        this.tasks = new LinkedHashMap<>();
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of(), tl.getTasks());
+    }
+
+    /*
+    getCompletedTasks()
+     */
+    @Test
+    public void getCompletedTasksWhenCompletedTasksExist() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", true);
+        tasks.put("Work out", true);
+        tasks.put("Do some programming", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of("Eat", "Work out"), tl.getCompletedTasks());
+    }
+
+    @Test
+    public void getCompletedTasksWhenCompletedTasksDontExist() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+        tasks.put("Work out", false);
+        tasks.put("Do some programming", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of(), tl.getCompletedTasks());
+    }
+
+    /*
+    getIncompleteTasks
+     */
+    @Test
+    public void getIncompleteTasksWhenIncompleteTasksExist() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", true);
+        tasks.put("Work out", true);
+        tasks.put("Do some programming", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of("Do some programming"), tl.getIncompleteTasks());
+    }
+
+    @Test
+    public void getIncompleteTasksWhenIncompleteTasksDontExist() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+        tasks.put("Work out", false);
+        tasks.put("Do some programming", true);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of("Eat", "Work out"), tl.getIncompleteTasks());
+    }
+
+    /*
+    changeTaskStatus(String taskName)
+     */
     @Test
-    public void exampleTest() {
-        String hello = "Hello";
-        Assertions.assertEquals("Hello", hello);
-        Assertions.assertNotEquals("Goodbye", hello);
+    public void changeTaskStatusWhenTaskExistsAndIsIncomplete() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(true, tl.changeTaskStatus("Eat"));
+        Assertions.assertEquals(true, tasks.get("Eat"));
+        Assertions.assertEquals(true, tl.changeTaskStatus("eat"));
+        Assertions.assertEquals(false, tasks.get("Eat"));
+    }
+
+    @Test
+    public void changeTaskStatusWhenTaskExistsAndIsCompleted() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", true);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(true, tl.changeTaskStatus("Eat"));
+        Assertions.assertEquals(false, tasks.get("Eat"));
+        Assertions.assertEquals(true, tl.changeTaskStatus("eat"));
+        Assertions.assertEquals(true, tasks.get("Eat"));
+    }
+
+    @Test
+    public void changeTaskStatusWhenTasksDoesNotExist() {
+        this.tasks = new LinkedHashMap<>();
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(false, tl.changeTaskStatus("Eat"));
+        Assertions.assertEquals(false, tl.changeTaskStatus("work out"));
+    }
+
+    /*
+    searchTask(String searchString)
+     */
+    @Test
+    public void searchTaskResultFound() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+        tasks.put("Work out", false);
+        tasks.put("Do some programming today", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of("Eat", "Work out", "Do some programming today"), tl.searchTask("t"));
+        Assertions.assertEquals(List.of("Work out", "Do some programming today"), tl.searchTask("o"));
+        Assertions.assertEquals(List.of("Eat"), tl.searchTask("eat"));
+    }
+
+    @Test
+    public void searchTaskResultNotFound() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+        tasks.put("Work out", false);
+        tasks.put("Do some programming today", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of("No tasks found"), tl.searchTask("z"));
+        Assertions.assertEquals(List.of("No tasks found"), tl.searchTask("zx"));
+        Assertions.assertEquals(List.of("No tasks found"), tl.searchTask(":123!"));
+    }
+
+    /*
+    removeTask(String taskName)
+     */
+    @Test
+    public void removeTaskThatExists() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+        tasks.put("Work out", false);
+        tasks.put("Do some programming today", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(true, tl.removeTask("Eat"));
+        Assertions.assertEquals(true, tl.removeTask("work out"));
+        Assertions.assertEquals(true, tl.removeTask("Do some programming Today"));
+    }
+
+    @Test
+    public void removeTaskThatDoesNotExist() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(false, tl.removeTask("work out"));
+        Assertions.assertEquals(false, tl.removeTask("Do some programming Today"));
+    }
+
+    /*
+    getAllTasksOrdered(boolean descending)
+     */
+    @Test
+    public void getAllTasksOrderedWhenNoTasksExist() {
+        this.tasks = new LinkedHashMap<>();
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of(), tl.getAllTasksOrdered(false));
+        Assertions.assertEquals(List.of(), tl.getAllTasksOrdered(true));
+    }
+
+    @Test
+    public void getAllTasksOrderedAscending() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+        tasks.put("Work out", false);
+        tasks.put("Do some programming today", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of("Do some programming today", "Eat", "Work out"), tl.getAllTasksOrdered(false));
+    }
+
+    @Test
+    public void getAllTasksOrderedDescending() {
+        this.tasks = new LinkedHashMap<>();
+        tasks.put("Eat", false);
+        tasks.put("Work out", false);
+        tasks.put("Do some programming today", false);
+
+        TodoList tl = new TodoList(tasks);
+
+        Assertions.assertEquals(List.of("Work out", "Eat", "Do some programming today"), tl.getAllTasksOrdered(true));
     }
 }
diff --git a/src/test/java/com/booleanuk/extension/TodoListExtensionTest.java b/src/test/java/com/booleanuk/extension/TodoListExtensionTest.java
new file mode 100644
index 000000000..4e5db6ed4
--- /dev/null
+++ b/src/test/java/com/booleanuk/extension/TodoListExtensionTest.java
@@ -0,0 +1,101 @@
+package com.booleanuk.extension;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.time.LocalDateTime;
+import java.util.ArrayList;
+import java.util.List;
+
+public class TodoListExtensionTest {
+
+    List<Task> tasks = null;
+
+    @Test
+    public void searchTaskByIdTaskNotExistsReturnsMessage() {
+        tasks = new ArrayList<>();
+
+        TodoListExtension tl = new TodoListExtension(tasks);
+
+        Assertions.assertEquals("No task found", tl.searchTaskById(1));
+    }
+
+    @Test
+    public void searchTaskByIdExistsReturnsTaskName() {
+        tasks = new ArrayList<>();
+        LocalDateTime taskDate = LocalDateTime.now();
+        tasks.add(new Task(1, "Eat", false, taskDate));
+
+        TodoListExtension tl = new TodoListExtension(tasks);
+
+        Assertions.assertEquals("Eat", tl.searchTaskById(1));
+    }
+
+    @Test
+    public void updateTaskNameByIdTaskNotExist() {
+        tasks = new ArrayList<>();
+
+        TodoListExtension tl = new TodoListExtension(tasks);
+
+        Assertions.assertEquals(false, tl.updateTaskNameById(1, "New name"));
+    }
+
+    @Test
+    public void updateTaskNameByIdTaskExists() {
+        tasks = new ArrayList<>();
+        LocalDateTime taskDate = LocalDateTime.now();
+        Task t = new Task(1, "Eat", false, taskDate);
+        tasks.add(t);
+
+        TodoListExtension tl = new TodoListExtension(tasks);
+
+        Assertions.assertEquals(true, tl.updateTaskNameById(1, "New name"));
+        Assertions.assertEquals("New name", t.getName());
+    }
+
+    @Test
+    public void changeTaskCompletionStatusTaskNotExist() {
+        tasks = new ArrayList<>();
+
+        TodoListExtension tl = new TodoListExtension(tasks);
+
+        Assertions.assertEquals(false, tl.changeTaskStatusById(1));
+    }
+
+    @Test
+    public void changeTaskCompletionStatusTaskExists() {
+        tasks = new ArrayList<>();
+        LocalDateTime taskDate = LocalDateTime.now();
+        Task t = new Task(1, "Eat", false, taskDate);
+        tasks.add(t);
+
+        TodoListExtension tl = new TodoListExtension(tasks);
+
+        Assertions.assertEquals(true, tl.changeTaskStatusById(1));
+        Assertions.assertTrue(t.isCompleted());
+        Assertions.assertEquals(true, tl.changeTaskStatusById(1));
+        Assertions.assertFalse(t.isCompleted());
+    }
+
+    @Test
+    public void getDateAndTimeByIdTaskNotExistReturnsNull() {
+        tasks = new ArrayList<>();
+
+        TodoListExtension tl = new TodoListExtension(tasks);
+
+        Assertions.assertNull(tl.getTaskCreationDate(1));
+    }
+
+    @Test
+    public void getDateAndTimeByIdTaskExists() {
+        tasks = new ArrayList<>();
+        LocalDateTime taskDate = LocalDateTime.now();
+        Task t = new Task(1, "Eat", false, taskDate);
+        tasks.add(t);
+
+        TodoListExtension tl = new TodoListExtension(tasks);
+
+        Assertions.assertEquals(taskDate, tl.getTaskCreationDate(1));
+    }
+
+}