Perovskite solar cells (PSCs) have attracted increasing interest from researchers due to their superb power conversion efficiencies. The intrinsic trap defects, unavoidably formed in the fabrication process, can induce nonradiative recombination, ion migration, I-V hysteresis, and instability of PSCs. However, trap defects are not always harmful. Fully understanding the fundamentals of trap defects as well as the passivation and doping strategies is helpful for further improving device efficiency and stability. This book summarizes the methods and strategies of defect passivation and material doping aiming to clearly describe the underlying mechanisms for high-efficiency and stable PSCs.