The STEP experiment is conceptually a modern version of Galileo's Free-Fall Experiment, in which Galileo is said to have dropped two weights from the Leaning Tower of Pisa to demonstrate that they fall at the same rate. Any difference in the ratio of inertial to passive gravitational mass of the weights results in a difference in the rate of fall. In STEP, the masses are in free fall in an orbit around the Earth, and if there is a violation of the Equivalence Principle they tend to follow slightly different orbits. The orbiting masses fall all the way around the Earth and never strike the ground, so that any small difference in the rate of fall can build a large displacement. STEP is designed to allow measuring displacements as small as 10^-13 centimeter.