Complex craters are characterized by a central uplift, a flat floor, and inward collapse around the rim (see Figure 3). On Earth, the diameter of complex impact craters is larger to about 2 to 4 km (Grieve, 1987); complex craters start to form on the Moon for diameters around 15-20 km (e.g., Howard, 1974). The central uplift is composed of rocks that originated below the crater floor. It represents a stratigraphic uplift of about one-tenth of the crater diameter (see, e.g., Melosh, 1989; Cintala and Grieve, 1998). Like for simple craters, complex craters are filled by a mixture of rock and mineral clasts, both shocked and unshocked, together with impact melt and material slumped in the crater from the walls and crater rim. Detailed study of the texture and composition of the different units forming the crater-fill breccia can permit the reconstruction of the successive phases of the crater formation and modification. This approach need however to be combined with numerical modeling (see Stöffler et al., 2004). Now learn about Cratering Mechanics.