The field of photovoltaic research has been lately dominated by the rapid evolution of low-cost and high-efficiency hybrid organic lead halide perovskite solar cells. Despite the considerable progress made in the efficiency of such devices, the achievement of long-term material and device stability remains a challenge. In this Perspective, insights into the role structural defects play in the stability of these perovskite absorbers are examined, highlighting the critical importance of vacancy type defects as the initiation sites for moisture-, oxygen-, and light-induced degradation and the approaches that are emerging to help overcome these issues. In the second part of the Perspective we consider the stability of tin-based perovskites. Here, the Sn4+ defects that arise upon material degradation are described along with the strategies being developed to enhance stability and decrease their formation. Finally, the discussion is extended to innately more stable layered tin-based perovskites, identifying them as a route to the development of efficient lead-free perovskite solar cells.