This chapter delves into the foundational role of simulation and testing techniques in the development and deployment of autonomous vehicle (AV) systems. As AVs must operate in a vast range of dynamic, unpredictable environments, simulation emerges as a crucial method for exploring and refining system behaviors before physical implementation. It provides a virtual proving ground where engineers can rigorously test and optimize the performance of perception, decision-making, and control systems under a wide variety of conditions—including scenarios that would be hazardous, cost-prohibitive, or nearly impossible to stage in the real world. From sudden pedestrian crossings to sensor malfunctions in adverse weather, simulation makes it feasible to test the full spectrum of operational situations an AV may encounter. When coupled with formal verification techniques and software validation protocols, simulation becomes a powerful enabler of safe, scalable, and continuous AV development, dramatically accelerating innovation while minimizing risk.