The emergence of lead halide perovskites as light absorbers has enabled low cost and efficient photovoltaics via a simple solution, high‐throughout process. However, the perovskite materials suffer from instability under various environmental stressors, including moisture, oxygen, heat, and irradiation, which heavily hinders the practical application of perovskite solar cells (PSCs). In this review, the structural and performance instability of perovskites and their degradation causes and mechanisms under different conditions are discussed. The state‐of‐the‐art strategies that stabilize the perovskite layer in solar cells are then summarized; moreover, the microscopic reasons for the improved environmental tolerance are elucidated. Due to the structural tunability of perovskites, the environmental tolerance, which is influenced by defects and extended imperfections in the polycrystalline films, can be enhanced by varying intrinsic factors of component, dimensionality, and crystallinity. Furthermore, the extrinsic factors to improve the environmental tolerance of perovskites are portrayed in terms of surface functionalized molecules, barrier layers, and encapsulants. The mechanism of each method in reducing the environmental sensitivity is highlighted to provide potential guidance in extending the lifetime of perovskite devices. Organic–inorganic hybrid perovskite solar cells (PSCs) have emerged as a promising technology for the next‐generation photovoltaic. The instability of PSCs is addressed as the major barrier to commercialization. This review covers the recent progress on strategies of stabilizing perovskite layers and discusses the microscopic mechanism for improved environmental tolerance. A personal perspective on the research direction of stable PSCs is provided.