It has been observed that every solar cell or solar module gradually loses its performance over time. In some cases the deterioration in performance is very slow, and in other cases it is very fast and happens because of degradation in solar cells and modules. It is a very serious issue and a big obstacle for long-term energy production from these systems. Degradation alone is keeping some of the efficient and really cost-effective solar cell technologies away from commercialization. Degradation issues must be overcome before successful introduction of these solar cell technologies into the market. There are lots of factors responsible for degradation of a solar cell and module. The degradation happens irrespective of its use but it is faster during its operation. In some cases the electrode materials decompose and diffuse into the active materials, whereas in other cases the active materials themselves decompose and lose their optical and electrical properties. Sunlight, temperature, humidity, and oxygen are some of the important parameters that accelerate degradation in solar cells. For reliable and sustainable solar cell technology, the prevention of degradation is a must and for this purpose the detection and understanding of degradation mechanisms in a solar cell is of great importance. This chapter covers the degradation mechanisms in different solar cell technologies and modules.