Organic-inorganic halide perovskites have attracted considerable attention in the past few years because of their remarkable performance in optoelectronic devices. However, long-term stability of the materials and devices remains the biggest challenge for realistic implementation of perovskite solar cells. Although significant efforts have been carried out on the causes of degradation at the device level, few measurements have been made at the surface analytic level to reveal the degradation mechanisms. In this Perspective, we review the effects of environmental factors, such as O2, water, and light, on the perovskite layer by monitoring the intrinsic electronic structure and compositional changes in different exposure tests. This work contributes in developing better understanding of the degradation mechanisms and improving the overall stability of perovskite solar cells.