Organic–inorganic perovskite materials in the form of nanocrystals and thin films have received enormous attention recently because of their unique optoelectronic properties such as high absorption coefficient, narrow and tunable emission bandwidth, high photoluminescence quantum yield, long exciton lifetime, and balanced charge transport properties. These properties have found applications in a number of important fields, including photovoltaic solar cells, light-emitting diodes, photodetectors, sensors, and lasers. However, the stability of the materials and devices is strongly affected by several factors such as water moisture, light, oxygen, temperature, solvent, and other materials in contact such as metal oxides used in devices. Defects, particularly those related to surface states, play a critical role in the stability as well as the performance of the perovskites. Various surface modification and defect passivation strategies have been developed to enhance stability and improve performance. We review some recent progress in the development of synthetic approaches to produce high-quality nanostructured and bulk film perovskites with controlled properties and functionalities. We also highlight the degradation mechanism and surface passivation approaches to address the issue of instability. To help gain deeper fundamental insight into mechanisms behind degradation and surface passivation, relevant properties, including structural, optical, electronic, and dynamic, are discussed and illustrated with proposed models.