diff --git a/src/test/java/com/example/usersapi/service/UserServiceTest.java b/src/test/java/com/example/usersapi/service/UserServiceTest.java
new file mode 100644
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+++ b/src/test/java/com/example/usersapi/service/UserServiceTest.java
@@ -0,0 +1,262 @@
+package com.example.usersapi.service;
+
+import com.example.usersapi.dto.CreateUserRequestDto;
+import com.example.usersapi.dto.UserPatchRequestDto;
+import com.example.usersapi.dto.UserResponseDto;
+import com.example.usersapi.exception.EntityNotFoundException;
+import com.example.usersapi.exception.RegistrationException;
+import com.example.usersapi.mapper.UserMapper;
+import com.example.usersapi.model.User;
+import com.example.usersapi.repository.UserRepository;
+import com.example.usersapi.service.impl.UserServiceImpl;
+import java.lang.reflect.Field;
+import java.time.LocalDate;
+import java.time.format.DateTimeFormatter;
+import java.util.List;
+import java.util.Optional;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.Mockito;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+@ExtendWith(MockitoExtension.class)
+class UserServiceTest {
+    private CreateUserRequestDto requestDto;
+    private User user;
+    private UserResponseDto responseDto;
+    private DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd/MM/yyyy");
+    @Mock
+    private UserRepository userRepository;
+    @Mock
+    private UserMapper userMapper;
+    @InjectMocks
+    private UserServiceImpl userService;
+
+    @BeforeEach
+    void init() throws Exception {
+        requestDto = new CreateUserRequestDto(
+                "user@gmail.com",
+                "Bob",
+                "Jackson",
+                "01/02/1990",
+                "414 Union Ave, Brooklyn, NY 11211",
+                "(111) 111-1111"
+        );
+        user = getUserFromCreateUserDto(1L, requestDto);
+        responseDto = getResponseDtoFromUser(user);
+
+        Field field = userService.getClass().getDeclaredField("minAge");
+        field.setAccessible(true);
+        field.setInt(userService, 18);
+    }
+
+    @Test
+    @DisplayName("Add a new used to db")
+    void createUser_ValidRequest_ShouldAddUser() {
+        Mockito.when(userMapper.toModel(requestDto)).thenReturn(user);
+        Mockito.when(userRepository.save(user)).thenReturn(user);
+        Mockito.when(userMapper.toDto(user)).thenReturn(responseDto);
+
+        UserResponseDto actual = userService.createUser(requestDto).getData();
+
+        Assertions.assertEquals(responseDto.getId(), actual.getId());
+        Assertions.assertEquals(responseDto.getBirthDate(), actual.getBirthDate());
+        Assertions.assertEquals(responseDto.getFirstName(), actual.getFirstName());
+        Assertions.assertEquals(responseDto.getAddress(), actual.getAddress());
+    }
+
+    @Test
+    @DisplayName("Add a new used to db. Throws Exception")
+    void createUser_AgeLessThan18_ThrowsException() {
+        CreateUserRequestDto requestDto = new CreateUserRequestDto(
+                "user@gmail.com",
+                "Bob",
+                "Jackson",
+                "01/02/2020",
+                null,
+                null
+        );
+
+        User user = getUserFromCreateUserDto(1L, requestDto);
+        Mockito.when(userMapper.toModel(requestDto)).thenReturn(user);
+
+        Assertions.assertThrows(RegistrationException.class,
+                () -> userService.createUser(requestDto));
+    }
+
+    @Test
+    @DisplayName("Update some user fields")
+    void patchUser_ValidRequest_ShouldUpdateUserField() {
+        String newLastName = "Moore";
+        Long userId = 1L;
+        User updatedUser = new User();
+        updatedUser.setId(userId);
+        updatedUser.setEmail(user.getEmail());
+        updatedUser.setLastName(newLastName);
+        updatedUser.setFirstName(user.getFirstName());
+        updatedUser.setAddress(user.getAddress());
+        updatedUser.setPhoneNumber(user.getPhoneNumber());
+        updatedUser.setBirthDate(user.getBirthDate());
+
+        UserResponseDto updatedResponseDto = getResponseDtoFromUser(updatedUser);
+
+        Mockito.when(userRepository.findById(userId)).thenReturn(Optional.of(user));
+        Mockito.when(userRepository.save(user)).thenReturn(user);
+        Mockito.when(userMapper.toDto(updatedUser)).thenReturn(updatedResponseDto);
+
+        UserPatchRequestDto userPatchRequestDto = new UserPatchRequestDto(
+                null, null, newLastName, null, null, null
+        );
+        UserResponseDto actual = userService.patchUser(userId, userPatchRequestDto).getData();
+        Assertions.assertEquals(newLastName, actual.getLastName());
+
+        String newEmail = "usersNewEmal@gmail.com";
+        updatedUser.setEmail(newEmail);
+        updatedResponseDto.setEmail(newEmail);
+        UserPatchRequestDto userPatchRequestDto1 = new UserPatchRequestDto(
+                newEmail, null, null, null, null, null
+        );
+        actual = userService.patchUser(userId, userPatchRequestDto1).getData();
+        Assertions.assertEquals(newEmail, actual.getEmail());
+    }
+
+    @Test
+    @DisplayName("Update all user's info. Throws exception")
+    void patchUser_WrongUserId_ShouldThrowException() {
+        Mockito.when(userRepository.findById(Mockito.anyLong())).thenReturn(Optional.empty());
+        Assertions.assertThrows(EntityNotFoundException.class,
+                () -> userService.patchUser(
+                        2L, new UserPatchRequestDto(null, null, null, null, null, null)
+                )
+        );
+    }
+
+    @Test
+    @DisplayName("Update all user's info")
+    void updateUser_ValidRequest_ShouldUpdateAllUserInfo() {
+        CreateUserRequestDto requestDto = new CreateUserRequestDto(
+                "userBob@gmail.com",
+                "Bobby",
+                "Spears",
+                "01/02/1989",
+                "4005 Dayrl Rd, Jacksonville, FL 32207",
+                "(222) 222-2222"
+        );
+        Long userId = 1L;
+        User updatedUser = getUserFromCreateUserDto(userId, requestDto);
+        UserResponseDto updatedResponseDto = getResponseDtoFromUser(updatedUser);
+
+        Mockito.when(userRepository.existsById(Mockito.anyLong())).thenReturn(true);
+        Mockito.when(userMapper.toModel(requestDto)).thenReturn(updatedUser);
+        Mockito.when(userMapper.toDto(updatedUser)).thenReturn(updatedResponseDto);
+        Mockito.when(userRepository.save(updatedUser)).thenReturn(updatedUser);
+
+        UserResponseDto actual = userService.updateUser(userId, requestDto).getData();
+        Assertions.assertEquals(requestDto.email(), actual.getEmail());
+        Assertions.assertEquals(requestDto.firstName(), actual.getFirstName());
+        Assertions.assertEquals(requestDto.lastName(), actual.getLastName());
+        Assertions.assertEquals(requestDto.address(), actual.getAddress());
+        Assertions.assertEquals(requestDto.phoneNumber(), actual.getPhoneNumber());
+        Assertions.assertEquals(requestDto.birthDate(), actual.getBirthDate());
+    }
+
+    @Test
+    @DisplayName("Search for users in birth dates range")
+    void searchByBirthDates_ValidRequest_ShouldReturnListOfUsersInDatesRange() {
+        User user2 = new User();
+        user2.setId(2L);
+        user2.setFirstName("Jane");
+        user2.setLastName("Smith");
+        user2.setEmail("user2@example.com");
+        user2.setBirthDate(LocalDate.of(1985, 5, 15));
+
+        User user3 = new User();
+        user3.setId(3L);
+        user3.setFirstName("Alice");
+        user3.setLastName("Johnson");
+        user3.setEmail("user3@example.com");
+        user3.setBirthDate(LocalDate.of(1988, 9, 20));
+
+        User user4 = new User();
+        user4.setId(4L);
+        user4.setFirstName("Bob");
+        user4.setLastName("Brown");
+        user4.setEmail("user4@example.com");
+        user4.setBirthDate(LocalDate.of(1995, 3, 10));
+
+        User user5 = new User();
+        user5.setId(5L);
+        user5.setFirstName("Emily");
+        user5.setLastName("Davis");
+        user5.setEmail("user5@example.com");
+        user5.setBirthDate(LocalDate.of(1992, 11, 28));
+
+        UserResponseDto responseDto2 = getResponseDtoFromUser(user2);
+        UserResponseDto responseDto3 = getResponseDtoFromUser(user3);
+        UserResponseDto responseDto4 = getResponseDtoFromUser(user4);
+        UserResponseDto responseDto5 = getResponseDtoFromUser(user5);
+
+        Mockito.when(userMapper.toDto(user)).thenReturn(responseDto);
+        Mockito.when(userMapper.toDto(user2)).thenReturn(responseDto2);
+        Mockito.when(userMapper.toDto(user3)).thenReturn(responseDto3);
+        Mockito.when(userMapper.toDto(user4)).thenReturn(responseDto4);
+        Mockito.when(userMapper.toDto(user5)).thenReturn(responseDto5);
+
+        String from = "02/03/1986";
+        String to = "10/03/1995";
+        Mockito.when(userRepository.findByBirthDateBetween(LocalDate.parse(from, formatter),
+                LocalDate.parse(to, formatter))).thenReturn(List.of(user, user3, user4));
+        List<UserResponseDto> expected = List.of(responseDto, responseDto3, responseDto4);
+        List<UserResponseDto> actual = userService.searchByBirthDates(from, to).getData();
+        Assertions.assertEquals(expected.size(), actual.size());
+        Assertions.assertTrue(actual.containsAll(expected));
+
+        from = "15/04/1985";
+        to = "10/03/2000";
+        Mockito.when(userRepository.findByBirthDateBetween(LocalDate.parse(from, formatter),
+                        LocalDate.parse(to, formatter)))
+                .thenReturn(List.of(user, user2, user3, user4, user5));
+        expected = List.of(responseDto, responseDto2, responseDto3, responseDto4, responseDto5);
+        actual = userService.searchByBirthDates(from, to).getData();
+        Assertions.assertEquals(expected.size(), actual.size());
+        Assertions.assertTrue(actual.containsAll(expected));
+
+        from = "15/04/1990";
+        to = "10/03/2003";
+        Mockito.when(userRepository.findByBirthDateBetween(LocalDate.parse(from, formatter),
+                LocalDate.parse(to, formatter))).thenReturn(List.of(user5));
+        expected = List.of(responseDto5);
+        actual = userService.searchByBirthDates(from, to).getData();
+        Assertions.assertEquals(expected.size(), actual.size());
+        Assertions.assertTrue(actual.containsAll(expected));
+    }
+
+    private User getUserFromCreateUserDto(Long userId, CreateUserRequestDto requestDto) {
+        User resulUser = new User();
+        resulUser.setId(userId);
+        resulUser.setEmail(requestDto.email());
+        resulUser.setFirstName(requestDto.firstName());
+        resulUser.setLastName(requestDto.lastName());
+        resulUser.setBirthDate(LocalDate.parse(requestDto.birthDate(), formatter));
+        resulUser.setAddress(requestDto.address());
+        resulUser.setPhoneNumber(requestDto.phoneNumber());
+        return resulUser;
+    }
+
+    private UserResponseDto getResponseDtoFromUser(User user) {
+        UserResponseDto userResponseDto = new UserResponseDto();
+        userResponseDto.setId(user.getId());
+        userResponseDto.setEmail(user.getEmail());
+        userResponseDto.setFirstName(user.getFirstName());
+        userResponseDto.setLastName(user.getLastName());
+        userResponseDto.setBirthDate(user.getBirthDate().format(formatter));
+        userResponseDto.setAddress(user.getAddress());
+        userResponseDto.setPhoneNumber(user.getPhoneNumber());
+        return userResponseDto;
+    }
+}