The rapid development of solar cells (SCs) based on organic-inorganic hybrid metal triiodide perovskite (MTP) materials holds great promise for next-generation photovoltaic devices. The demonstrated power conversion efficiency of the SCs based on MTP (PSCs for short) has reached over 20%. An MTP material is a kind of soft ionic solid semiconductor. The intrinsic optoelectronic properties of MTP are greatly determined by several factors, such as the crystalline phase, doping type, impurities, elemental composition, and defects in its crystal structure. In the development of PSCs, a good understanding and smart engineering of the defects in MTP have been demonstrated to be a key factor for the fabrication of high-efficiency PSCs. In this review, we start with a brief introduction to the types of defects and the mechanisms for their formation in MTP. Then, the positive and negative impacts of defects on the important optoelectronic features of MTP are presented. The optoelectronic properties mainly include charge recombination, charge transport, ion migration, and structural stability. Moreover, commonly used techniques for the characterization of the defects in MTP are systematically summarized. Recent progress on the state-of-the-art defect engineering approaches for the optimization of PSC devices is also summarized, and we also provide some perspectives on the development of high-efficiency PSCs with long-term stability through the optimization of the defects in MTP.