Metal halide perovskites are an emerging materials platform for optoelectronic, spintronic, and thermoelectric applications. The field of perovskite materials and devices has progressed rapidly over the past decade. For halide perovskite materials, a range of physical and chemical properties such as crystal structure, bandgap, charge carrier density, and stability that govern the device functionalities are critically determined by the oxidation states of the B-site metal ions. However, such an important mechanistic connection unique to halide perovskites is not well established, limiting the pace of development in this area. In this review, we identify the roles of metal oxidation states in perovskite semiconductors. The redox reactions leading to these states, and their effects on the materials properties, are clarified. Finally, we suggest routes to improving device efficiency and stability from the perspective of oxidation state control.