diff --git a/Sources/UserInterfaceVariations/UIVariationEnvironment.swift b/Sources/UserInterfaceVariations/UIVariationEnvironment.swift
new file mode 100644
index 0000000..e247e38
--- /dev/null
+++ b/Sources/UserInterfaceVariations/UIVariationEnvironment.swift
@@ -0,0 +1,72 @@
+import UIKit
+
+public protocol UIVariationEnvironment { }
+
+extension UIVariationEnvironment where Self: UITraitEnvironment {
+    
+    public var variations: NSMutableArray {
+        get {
+            let key = Unmanaged.passUnretained(association).toOpaque()
+            if let array = objc_getAssociatedObject(self, key) as! NSMutableArray? { return array }
+            let array = NSMutableArray()
+            objc_setAssociatedObject(self, key, array, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
+            return array
+        }
+    }
+    
+    public func addVariation<Object, Value>(_ variation: UIVariation<Object, Value>) {
+        variation.traitEnvironment = self
+        variations.add(variation)
+        variation.applyIfMatchesTraitEnvironment()
+    }
+    
+    public func removeVariation<Object, Value>(_ variation: UIVariation<Object, Value>) {
+        variations.remove(variation)
+        variation.traitEnvironment = nil
+    }
+    
+    public func addVariation<Object: AnyObject, Value>(for object: Object,
+                                                       property: ReferenceWritableKeyPath<Object, Value>,
+                                                       sizeClassDimension: SizeClassDimension,
+                                                       whenCompact: Value) {
+        switch sizeClassDimension {
+        case .horizontal:
+            addVariation(UIVariation<Object, Value>(object: object,
+                                                    keyPath: property,
+                                                    value: object[keyPath: property],
+                                                    horizontalSizeClass: .regular,
+                                                    verticalSizeClass: nil))
+            addVariation(UIVariation<Object, Value>(object: object,
+                                                    keyPath: property,
+                                                    value: whenCompact,
+                                                    horizontalSizeClass: .compact,
+                                                    verticalSizeClass: nil))
+        case .vertical:
+            addVariation(UIVariation<Object, Value>(object: object,
+                                                    keyPath: property,
+                                                    value: object[keyPath: property],
+                                                    horizontalSizeClass: nil,
+                                                    verticalSizeClass: .regular))
+            addVariation(UIVariation<Object, Value>(object: object,
+                                                    keyPath: property,
+                                                    value: whenCompact,
+                                                    horizontalSizeClass: nil,
+                                                    verticalSizeClass: .compact))
+        }
+    }
+    
+    /// Call this method when trait collection changes to keep variations reacting to them.
+    ///
+    ///     override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
+    ///         super.traitCollectionDidChange(previousTraitCollection)
+    ///         activateVariationsMatchingTraitEnvironment()
+    ///     }
+    ///
+    public func activateVariationsMatchingTraitEnvironment() {
+        for variation in variations as! [UIVariationApplying] {
+            variation.applyIfMatchesTraitEnvironment()
+        }
+    }
+}
+
+private let association = NSObject()
diff --git a/Tests/UserInterfaceVariationsTests/UIVariationEnvironmentTests.swift b/Tests/UserInterfaceVariationsTests/UIVariationEnvironmentTests.swift
new file mode 100644
index 0000000..8e46084
--- /dev/null
+++ b/Tests/UserInterfaceVariationsTests/UIVariationEnvironmentTests.swift
@@ -0,0 +1,101 @@
+import XCTest
+@testable import UserInterfaceVariations
+
+final class UIVariationEnvironmentTests: XCTestCase {
+    
+    let label = UILabel()
+    let valueWhenCompact = "value when compact"
+    let valueWhenRegular = "value when regular"
+    
+    override func setUp() {
+        label.text = nil
+    }
+    
+    func test_When_variation_matches_traitEnvironment_Then_it_is_applied() {
+        let variationEnvironment = UIVariationEnvironmentFake(traitCollection: UITraitCollection(horizontalSizeClass: .compact))
+        let variation = UIVariation(object: label,
+                                    keyPath: \.text,
+                                    value: valueWhenCompact,
+                                    horizontalSizeClass: .compact,
+                                    verticalSizeClass: nil)
+        variationEnvironment.addVariation(variation)
+        XCTAssertEqual(label.text, variation.value)
+    }
+    
+    func test_adding_variation_convenience_method_for_sizeClassDimension_horizontal() {
+        label.text = valueWhenRegular
+        
+        let variationEnvironment = UIVariationEnvironmentFake(traitCollection: UITraitCollection(horizontalSizeClass: .regular))
+        variationEnvironment.addVariation(for: label,
+                                          property: \.text,
+                                          sizeClassDimension: .horizontal,
+                                          whenCompact: valueWhenCompact)
+        
+        let variation0 = variationEnvironment.variations[0] as! UIVariation<UILabel, String?>
+        XCTAssertTrue(variation0.object === label)
+        XCTAssertTrue(variation0.property == \.text)
+        XCTAssertTrue(variation0.value == valueWhenRegular)
+        XCTAssertTrue(variation0.horizontalSizeClass == .regular)
+        XCTAssertTrue(variation0.verticalSizeClass == nil)
+        
+        let variation1 = variationEnvironment.variations[1] as! UIVariation<UILabel, String?>
+        XCTAssertTrue(variation1.object === label)
+        XCTAssertTrue(variation1.property == \.text)
+        XCTAssertTrue(variation1.value == valueWhenCompact)
+        XCTAssertTrue(variation1.horizontalSizeClass == .compact)
+        XCTAssertTrue(variation1.verticalSizeClass == nil)
+    }
+    
+    func test_adding_variation_convenience_method_for_sizeClassDimension_vartical() {
+        label.text = valueWhenRegular
+        
+        let variationEnvironment = UIVariationEnvironmentFake(traitCollection: UITraitCollection(horizontalSizeClass: .regular))
+        variationEnvironment.addVariation(for: label,
+                                          property: \.text,
+                                          sizeClassDimension: .vertical,
+                                          whenCompact: valueWhenCompact)
+        
+        let variation0 = variationEnvironment.variations[0] as! UIVariation<UILabel, String?>
+        XCTAssertTrue(variation0.object === label)
+        XCTAssertTrue(variation0.property == \.text)
+        XCTAssertTrue(variation0.value == valueWhenRegular)
+        XCTAssertTrue(variation0.horizontalSizeClass == nil)
+        XCTAssertTrue(variation0.verticalSizeClass == .regular)
+        
+        let variation1 = variationEnvironment.variations[1] as! UIVariation<UILabel, String?>
+        XCTAssertTrue(variation1.object === label)
+        XCTAssertTrue(variation1.property == \.text)
+        XCTAssertTrue(variation1.value == valueWhenCompact)
+        XCTAssertTrue(variation1.horizontalSizeClass == nil)
+        XCTAssertTrue(variation1.verticalSizeClass == .compact)
+    }
+    
+    func test_When_verticalSizeClass_in_traitCollection_changes_Then_variation_is_applied() {
+        let variationEnvironment = UIVariationEnvironmentFake(traitCollection: UITraitCollection(verticalSizeClass: .regular))
+        label.text = valueWhenRegular
+        variationEnvironment.addVariation(for: label,
+                                          property: \.text,
+                                          sizeClassDimension: .vertical,
+                                          whenCompact: valueWhenCompact)
+        variationEnvironment.traitCollection = UITraitCollection(verticalSizeClass: .compact)
+        XCTAssertEqual(label.text, valueWhenCompact)
+    }
+}
+
+class UIVariationEnvironmentFake: NSObject, UITraitEnvironment {
+    
+    var traitCollection = UITraitCollection() {
+        didSet { traitCollectionDidChange(oldValue) }
+    }
+    
+    init(traitCollection: UITraitCollection) {
+        self.traitCollection = traitCollection
+        super.init()
+    }
+    
+    func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
+        activateVariationsMatchingTraitEnvironment()
+    }
+}
+
+extension UIVariationEnvironmentFake: UIVariationEnvironment { }