The need to determine relative ages of materials and surfaces on moons and planets other than the Earth has resulted in the development of dating techniques that are based on the density or the morphology of craters and that supplement the classical techniques of physical stratigraphy. As is the case with the fossil-based relative time scale on Earth, crater-based relative ages can, in principal, be calibrated with radiometric ages of returned samples. Relative ages determined by crater density or crater morphology rest on a small number of basic assumptions concerning the morphology of fresh craters, the randomness of crater-formation processes, and the rates and areal constancy of crater-degradation processes. The validity of these assumptions varies from planet to planet. Despite the problems and controversies that inevitably accompany the development of major new techniques, the basic principles underlying the use of craters to determine relative ages are well established and logically sound.