Impact craters are common geological structures widely observed on all rocky surfaces in the Solar System. Other than on the Earth, impact craters are among the dominant features in geological records. They form by rocks from space striking lunar, asteroidal, and planetary surfaces at high speeds and delivering extensive impact energy. Such a violent process permanently changes the structure of their crusts and interiors. Impacts have played a vital role in the evolution of rocky planetary surfaces over geological timescales and as such are witnesses of the geological epoch they formed in. Tracking their formation across geological timelines provide clues to the planet's geological evolution. Very large impacts were common during the early evolution of the Solar System. While the frequency and scale of impact catastrophes have decreased with the age of the Solar System, impact bombardment still plays a role in planetary resurfacing. This talk will explain the impact cratering process and how numerical impact simulations help answer fundamental questions in planetary sciences by validating models against space mission data, including the NASA InSight currently operating on Mars.