diff --git a/Sources/PenguinStructures/Concatenation.swift b/Sources/PenguinStructures/Concatenation.swift
new file mode 100644
index 00000000..41842ba6
--- /dev/null
+++ b/Sources/PenguinStructures/Concatenation.swift
@@ -0,0 +1,203 @@
+// Copyright 2020 Penguin Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+
+/// A collection that is all the elements of one collection followed by all the elements of a second
+/// collection.
+public struct Concatenation<First: Collection, Second: Collection>: Collection 
+where First.Element == Second.Element {
+  /// The elements in `self`.
+  public typealias Element = First.Element
+  /// The collection whose elements appear first.
+  public var first: First
+  /// The collection whose elements appear second.
+  public var second: Second
+
+  /// Concatenates `first` with `second`.
+  public init(_ first: First, _ second: Second) {
+    self.first = first
+    self.second = second
+  }
+
+  /// A position in a `Concatenation`.
+  public struct Index: Comparable {
+    /// A position into one of the two underlying collections.
+    @usableFromInline
+    var position: Either<First.Index, Second.Index>
+
+    /// Creates a new index into the first underlying collection.
+    @usableFromInline
+    internal init(first i: First.Index) {
+      self.position = .a(i)
+    }
+
+    /// Creates a new index into the first underlying collection.
+    @usableFromInline
+    internal init(second i: Second.Index) {
+      self.position = .b(i)
+    }
+
+    /// Returns `true` iff `lhs` precedes `rhs`.
+    public static func < (lhs: Self, rhs: Self) -> Bool {
+      return lhs.position < rhs.position
+    }
+  }
+
+  /// The position of the first element, or `endIndex` if `self.isEmpty`
+  @inlinable
+  public var startIndex: Index {
+    if !first.isEmpty { return Index(first: first.startIndex) }
+    return Index(second: second.startIndex)
+  }
+
+  /// The collection’s “past the last” position—that is, the position one greater than the last
+  /// valid subscript argument.
+  @inlinable
+  public var endIndex: Index { Index(second: second.endIndex) }
+
+  /// Returns the next index after `i`.
+  public func index(after i: Index) -> Index {
+    switch i.position {
+    case .a(let index):
+      let newIndex = first.index(after: index)
+      guard newIndex != first.endIndex else { return Index(second: second.startIndex) }
+      return Index(first: newIndex)
+    case .b(let index):
+      return Index(second: second.index(after: index))
+    }
+  }
+
+  /// Accesses the element at `i`.
+  @inlinable
+  public subscript(i: Index) -> Element {
+    switch i.position {
+    case .a(let index): return first[index]
+    case .b(let index): return second[index]
+    }
+  }
+
+  /// The number of elements in `self`.
+  @inlinable
+  public var count: Int { first.count + second.count }
+
+  /// True iff `self` contains no elements.
+  @inlinable
+  public var isEmpty: Bool { first.isEmpty && second.isEmpty }
+
+  /// Returns the distance between two indices.
+  @inlinable
+  public func distance(from start: Index, to end: Index) -> Int {
+    switch (start.position, end.position) {
+    case (.a(let start), .a(let end)):
+      return first.distance(from: start, to: end)
+    case (.a(let start), .b(let end)):
+      return first.distance(from: start, to: first.endIndex) + second.distance(from: second.startIndex, to: end)
+    case (.b(let start), .a(let end)):
+      return second.distance(from: start, to: second.startIndex) + first.distance(from: first.endIndex, to: end)
+    case (.b(let start), .b(let end)):
+      return second.distance(from: start, to: end)
+    }
+  }
+}
+
+extension Concatenation: BidirectionalCollection
+where First: BidirectionalCollection, Second: BidirectionalCollection {
+  /// Returns the next position before `i`.
+  @inlinable
+  public func index(before i: Index) -> Index {
+    switch i.position {
+    case .a(let index): return Index(first: first.index(before: index))
+    case .b(let index):
+      if index == second.startIndex {
+        return Index(first: first.index(before: first.endIndex))
+      }
+      return Index(second: second.index(before: index))
+    }
+  }
+}
+
+extension Concatenation: RandomAccessCollection
+  where First: RandomAccessCollection, Second: RandomAccessCollection
+{
+  @inlinable
+  public func index(_ i: Index, offsetBy n: Int) -> Index {
+    if n == 0 { return i }
+    if n < 0 { return offsetBackward(i, by: n) }
+    return offsetForward(i, by: n)
+  }
+
+  @usableFromInline
+  func offsetForward(_ i: Index, by n: Int) -> Index {
+    switch i.position {
+    case .a(let index):
+      let d = first.distance(from: index, to: first.endIndex)
+      if n < d {
+        return Index(first: first.index(index, offsetBy: n))
+      } else {
+        return Index(second: second.index(second.startIndex, offsetBy: n - d))
+      }
+    case .b(let index):
+      return Index(second: second.index(index, offsetBy: n))
+    }
+  }
+
+  @usableFromInline
+  func offsetBackward(_ i: Index, by n: Int) -> Index {
+    switch i.position {
+    case .a(let index):
+      return Index(first: first.index(index, offsetBy: n))
+    case .b(let index):
+      let d = second.distance(from: second.startIndex, to: index)
+      if -n <= d {
+        return Index(second: second.index(index, offsetBy: n))
+      } else {
+        return Index(first: first.index(first.endIndex, offsetBy: n + d))
+      }
+    }
+  }
+}
+
+extension Concatenation: MutableCollection
+  where First: MutableCollection, Second: MutableCollection
+{
+  /// Accesses the element at `i`.
+  @inlinable
+  public subscript(i: Index) -> Element {
+    get {
+      switch i.position {
+      case .a(let index): return first[index]
+      case .b(let index): return second[index]
+      }
+    }
+    set {
+      switch i.position {
+      case .a(let index): first[index] = newValue
+      case .b(let index): second[index] = newValue
+      }
+    }
+  }
+}
+
+
+extension Collection {
+  /// Returns a collection containing the elements of `self` followed by the elements of `other`.
+  ///
+  /// - Complexity: O(1)
+  @inlinable
+  public func concatenated<Other: Collection>(to other: Other) -> Concatenation<Self, Other>
+    where Other.Element == Element
+  {
+    return Concatenation(self, other)
+  }
+}
diff --git a/Tests/PenguinStructuresTests/ConcatenationTests.swift b/Tests/PenguinStructuresTests/ConcatenationTests.swift
new file mode 100644
index 00000000..9f560ec9
--- /dev/null
+++ b/Tests/PenguinStructuresTests/ConcatenationTests.swift
@@ -0,0 +1,108 @@
+// Copyright 2020 Penguin Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import PenguinStructures
+import XCTest
+
+final class ConcatenationTests: XCTestCase {
+
+  func testInit() {
+    XCTAssert(Concatenation(0..<10, 10..<20).elementsEqual(0..<20))
+  }
+
+  func testConcatenated() {
+    XCTAssert((0..<10).concatenated(to: 10..<20).elementsEqual(0..<20))
+  }
+
+  func testConditionalConformances() {
+    let a3 = 0..<10, b3 = 10..<20, concatenated = 0..<20
+    let a2 = AnyBidirectionalCollection(a3), b2 = AnyBidirectionalCollection(b3)
+    let a1 = AnyCollection(a3), b1 = AnyCollection(b3)
+    let j11 = a1.concatenated(to: b1)
+    XCTAssertFalse(j11.isBidirectional)
+    j11.checkCollectionSemantics(expectedValues: concatenated)
+
+    let j12 = a1.concatenated(to: b2)
+    XCTAssertFalse(j12.isBidirectional)
+    j12.checkCollectionSemantics(expectedValues: concatenated)
+
+    let j13 = a1.concatenated(to: b3)
+    XCTAssertFalse(j13.isBidirectional)
+    j13.checkCollectionSemantics(expectedValues: concatenated)
+
+    let j21 = a2.concatenated(to: b1)
+    XCTAssertFalse(j21.isBidirectional)
+    j21.checkCollectionSemantics(expectedValues: concatenated)
+
+    let j22 = a2.concatenated(to: b2)
+    XCTAssert(j22.isBidirectional)
+    XCTAssertFalse(j22.isRandomAccess)
+    j22.checkBidirectionalCollectionSemantics(expectedValues: concatenated)
+
+    let j23 = a2.concatenated(to: b3)
+    XCTAssert(j23.isBidirectional)
+    XCTAssertFalse(j23.isRandomAccess)
+    j23.checkBidirectionalCollectionSemantics(expectedValues: concatenated)
+
+    let j31 = a3.concatenated(to: b1)
+    XCTAssertFalse(j31.isBidirectional)
+    j31.checkCollectionSemantics(expectedValues: concatenated)
+
+    let j32 = a3.concatenated(to: b2)
+    XCTAssert(j32.isBidirectional)
+    XCTAssertFalse(j32.isRandomAccess)
+    j32.checkBidirectionalCollectionSemantics(expectedValues: concatenated)
+
+    let j33 = a3.concatenated(to: b3)
+    XCTAssert(j33.isRandomAccess)
+    j33.checkRandomAccessCollectionSemantics(expectedValues: concatenated)
+  }
+
+  func testConcatenateSetToArray() {
+    let s = Set(["1", "2", "3"])
+    let c = s.concatenated(to: ["10", "11", "12"])
+    c.checkCollectionSemantics(expectedValues: Array(s) + ["10", "11", "12"])
+  }
+
+  func testConcatenateRanges() {
+    let c = (0..<3).concatenated(to: 3...6)
+    c.checkRandomAccessCollectionSemantics(expectedValues: 0...6)
+  }
+
+  func testConcatenateEmptyPrefix() {
+    let c = (0..<0).concatenated(to: [1, 2, 3])
+    c.checkRandomAccessCollectionSemantics(expectedValues: 1...3)
+  }
+
+  func testConcatenateEmptySuffix() {
+    let c = (1...3).concatenated(to: 1000..<1000)
+    c.checkRandomAccessCollectionSemantics(expectedValues: [1, 2, 3])
+  }
+
+  func testMutableCollection() {
+    let a = Array(0..<10), b = Array(10..<20)
+    var j = a.concatenated(to: b)
+    j.checkMutableCollectionSemantics(source: 20..<40)
+  }
+
+  static var allTests = [
+    ("testInit", testInit),
+    ("testConditionalConformances", testConditionalConformances),
+    ("testConcatenateSetToArray", testConcatenateSetToArray),
+    ("testConcatenateRanges", testConcatenateRanges),
+    ("testConcatenateEmptyPrefix", testConcatenateEmptyPrefix),
+    ("testConcatenateEmptySuffix", testConcatenateEmptySuffix),
+    ("testMutableCollection", testMutableCollection),
+  ]
+}
diff --git a/Tests/PenguinStructuresTests/XCTestManifests.swift b/Tests/PenguinStructuresTests/XCTestManifests.swift
index 0523eddd..dc738b64 100644
--- a/Tests/PenguinStructuresTests/XCTestManifests.swift
+++ b/Tests/PenguinStructuresTests/XCTestManifests.swift
@@ -23,6 +23,7 @@ import XCTest
       testCase(ArrayStorageExtensionTests.allTests),
       testCase(ArrayStorageTests.allTests),
       testCase(CollectionAlgorithmTests.allTests),
+      testCase(ConcatenationTests.allTests),
       testCase(DequeTests.allTests),
       testCase(DoubleEndedBufferTests.allTests),
       testCase(EitherCollectionTests.allTests),