Collisions between celestial materials are a fundamental process in the formation and evolution of our Solar System. Impact structures are the most common landform on Solar System bodies that have solid surfaces. On most planetary bodies, impact cratering has long been the dominant process modifying surface topography. Compared with the other geological processes that occur in lithospheres of planetary bodies, impact cratering features temperatures, pressure, and strain rate that are higher by several orders of magnitude, resulting in widespread vaporization, melting, metamorphism, and deformation. Impact cratering is a transient process, but it changes the interior and exterior structures of planetary bodies by injecting energy and can profoundly affect the evolution of planetary systems. Due to continuous impact cratering, impact craters have accumulated on planetary bodies since their birth, so the spatial density of impact craters reflects the impact history. In the inner Solar System, the impact flux was much higher at ~3. 8 Ga, but it has long been debated whether many impact basins were formed catastrophically within a short time. The impact flux has been more- or- less constant since ~3. 8 Ga, but calibration samples that have unambiguous geological affiliations are lacking. On a given planetary body, the spatial density of impact craters can be used to evaluate relative ages of different geological units. Samples recovered from the Moon have been used to reconstruct the formation rates of different- sized impact craters, so crater statistics can be used to estimate absolute model ages for lunar surfaces. Meanwhile, the Moon and terrestrial planets may have experienced the same impact flux, so formation rates of lunar craters can be translated to the other bodies based on assumed ratios of impact flux and cratering scaling laws. As a basic technique of planetary geology, crater statistics has been a major method in the study of impact history and remote estimation of relative and absolute model ages for planetary surfaces. While the reliability of this technique has been verified in many trial tests, both the theoretical basis and technical details of this technique have uncertainties. Calibrating this technique is an important target in both sample return missions and planetary science.