diff --git a/.releases/4.18.0.md b/.releases/4.18.0.md
index 1bd63e3..ac22d75 100644
--- a/.releases/4.18.0.md
+++ b/.releases/4.18.0.md
@@ -1,5 +1,6 @@
 **New Features**
 
 * Added a `Changes` specification.
+* Added a `Crosses` specification.
 * Added a `Null` specification.
 * Added an `Undefined` specification.
\ No newline at end of file
diff --git a/specifications/Crosses.js b/specifications/Crosses.js
new file mode 100644
index 0000000..12b6631
--- /dev/null
+++ b/specifications/Crosses.js
@@ -0,0 +1,57 @@
+const assert = require('./../lang/assert');
+	is = require('./../lang/is');
+
+const Specification = require('./Specification');
+
+module.exports = (() => {
+	'use strict';
+
+	/**
+	 * A stateful {@link Specification} that passes when the value of the data item
+	 * crosses a value passed to the constructor. The specification will never pass
+	 * on the first evaluation. Instead, the first data item is used to determine
+	 * if the value is currently greater than (or less than) the threshold value
+	 * (passed to the constructor). This determines if the passing condition means
+	 * the value must be less than (or greater than) the threshold.
+	 *
+	 * @public
+	 * @extends {Specification}
+	 * @param {Number} threshold
+	 */
+	class CrossesSpecification extends Specification {
+		constructor(threshold) {
+			super();
+
+			assert.argumentIsRequired(threshold, 'threshold', Number);
+
+			this._threshold = threshold;
+
+			this._previous = null;
+		}
+
+		_evaluate(data) {
+			if (!is.number(data)) {
+				return false;
+			}
+
+			const current = data;
+			const previous = this._previous;
+
+			const crossed = previous !== null &&
+				(
+					(previous > this._threshold && !(current > this._threshold)) ||
+					(previous < this._threshold && !(current < this._threshold))
+				);
+
+			this._previous = current;
+
+			return crossed;
+		}
+
+		toString() {
+			return '[CrossesSpecification]';
+		}
+	}
+
+	return CrossesSpecification;
+})();
\ No newline at end of file
diff --git a/test/specs/specifications/CrossesSpec.js b/test/specs/specifications/CrossesSpec.js
new file mode 100644
index 0000000..c31cc29
--- /dev/null
+++ b/test/specs/specifications/CrossesSpec.js
@@ -0,0 +1,189 @@
+const Crosses = require('./../../../specifications/Crosses');
+
+describe('When a Crosses specification is initialized with a threshold of 1000', () => {
+	'use strict';
+
+	let specification;
+
+	beforeEach(() => {
+		specification = new Crosses(1000);
+	});
+
+	describe('and the first value evaluated is 900', () => {
+		let r1;
+
+		beforeEach(() => {
+			r1 = specification.evaluate(900);
+		});
+
+		it('should not pass', () => {
+			expect(r1).toEqual(false);
+		});
+
+		describe('and the second value evaluated is 1100', () => {
+			let r2;
+
+			beforeEach(() => {
+				r2 = specification.evaluate(1100);
+			});
+
+			it('should pass', () => {
+				expect(r2).toEqual(true);
+			});
+
+			describe('and the third value evaluated is 999', () => {
+				let r3;
+
+				beforeEach(() => {
+					r3 = specification.evaluate(999);
+				});
+
+				it('should pass', () => {
+					expect(r3).toEqual(true);
+				});
+			});
+
+			describe('and the third value evaluated is 1001', () => {
+				let r3;
+
+				beforeEach(() => {
+					r3 = specification.evaluate(1001);
+				});
+
+				it('should not pass', () => {
+					expect(r3).toEqual(false);
+				});
+			});
+		});
+
+		describe('and the second value evaluated is 950', () => {
+			let r2;
+
+			beforeEach(() => {
+				r2 = specification.evaluate(950);
+			});
+
+			it('should not pass', () => {
+				expect(r2).toEqual(false);
+			});
+		});
+	});
+
+	describe('and the first value evaluated is 1200', () => {
+		let r1;
+
+		beforeEach(() => {
+			r1 = specification.evaluate(1200);
+		});
+
+		it('should not pass', () => {
+			expect(r1).toEqual(false);
+		});
+
+		describe('and the second value evaluated is 1100', () => {
+			let r2;
+
+			beforeEach(() => {
+				r2 = specification.evaluate(1100);
+			});
+
+			it('should not pass', () => {
+				expect(r2).toEqual(false);
+			});
+		});
+
+		describe('and the second value evaluated is 950', () => {
+			let r2;
+
+			beforeEach(() => {
+				r2 = specification.evaluate(950);
+			});
+
+			it('should pass', () => {
+				expect(r2).toEqual(true);
+			});
+		});
+	});
+});
+
+describe('When a Crosses specification is initialized with a threshold of zero', () => {
+	'use strict';
+
+	let specification;
+
+	beforeEach(() => {
+		specification = new Crosses(0);
+	});
+
+	describe('and the first value evaluated is 1', () => {
+		let r1;
+
+		beforeEach(() => {
+			r1 = specification.evaluate(1);
+		});
+
+		it('should not pass', () => {
+			expect(r1).toEqual(false);
+		});
+
+		describe('and the second value evaluated is -1', () => {
+			let r2;
+
+			beforeEach(() => {
+				r2 = specification.evaluate(-1);
+			});
+
+			it('should pass', () => {
+				expect(r2).toEqual(true);
+			});
+		});
+
+		describe('and the second value evaluated is 0.5', () => {
+			let r2;
+
+			beforeEach(() => {
+				r2 = specification.evaluate(0.5);
+			});
+
+			it('should not pass', () => {
+				expect(r2).toEqual(false);
+			});
+		});
+	});
+
+	describe('and the first value evaluated is -1', () => {
+		let r1;
+
+		beforeEach(() => {
+			r1 = specification.evaluate(-1);
+		});
+
+		it('should not pass', () => {
+			expect(r1).toEqual(false);
+		});
+
+		describe('and the second value evaluated is -0.5', () => {
+			let r2;
+
+			beforeEach(() => {
+				r2 = specification.evaluate(-0.5);
+			});
+
+			it('should not pass', () => {
+				expect(r2).toEqual(false);
+			});
+		});
+
+		describe('and the second value evaluated is 1', () => {
+			let r2;
+
+			beforeEach(() => {
+				r2 = specification.evaluate(1);
+			});
+
+			it('should pass', () => {
+				expect(r2).toEqual(true);
+			});
+		});
+	});
+});