Thin film CIGS solar cells and individual layers within these solar cells have been tested in order to assess their long term stability. Alongside with the execution of standard tests, in which elevated temperatures and humidity levels are used, the solar cells have also been exposed to a combination of elevated temperature and humidity and illumination, which also allowed in-situ analysis of the changes in the electrical parameters. Additionally, the samples have been tested in the presence of water and various atmospheric species, like CO2, nitrogen and oxygen, in order to assess the impact of these species. Based on these experiments, it was concluded that CIGS solar cells can rapidly lose efficiency due to the migration of sodium, which occurs when exposed to illumination and water vapor. It was also observed that the transparent top electrode, consisting of ZnO:Al degraded rapidly in the presence of a combination of water and CO2, while it is stable in the presence of water combined with e.g. N2 and O2. The thesis also contains an extensive literature study on the stability of CIGS solar cells and a study on the temperature dependency of these solar cells.