Galileo described his experiment with an inclined plane in his book, Two New Sciences. In this work Galileo was operating within a research tradition in physics known as "impetus." This tradition, begun in the 6th century CE with John Philoponos, sought to explain motion. Galileo constructed his inclined plane in order to slow down free fall to speeds which might accurately be measured. This OER explores the role of Galileo's inclined plane in the development of physics.