Copper indium gallium selenide (CIGS) based solar cells are receiving worldwide attention for solar power generation. They are efficient thin film solar cells that have achieved 22.8% efficiency comparable to crystalline silicon (c-Si) wafer based solar cells. For a production capacity of 1000 MW y⁻¹ with 15% module efficiency, the CIGS module production cost is expected to be $0.34 W⁻¹. For CIGS cells over glass, a graded bandgap high temperature deposition process has been established, however, this process has not been established for CIGS over flexible polymer substrates which is a low temperature process. For small area devices, the main focus is precise control over CIGS film stoichiometry and efficiency. For industrial production, apart from stoichiometry and efficiency, low-cost, reproducibility, high-throughput and process tolerance are of much importance in commercializing the technology. Due to process complexity, CIGS module production is lagging behind that of cadmium telluride (CdTe) modules. In this review article, the working mechanism of CIGS solar cells with a back surface field, the importance of developing CIGS solar cells, and the limitations for their commercialization are discussed. CIGS solar cells are compared with c-Si solar cells. After briefly reviewing the history of the chalcopyrite alloy system, graded bandgaps, effects of sodium distribution in CIGS layers, growth of CIGS layers using various techniques, role of buffer layer/transparent conducting oxides, CdS free buffer layers, concerns related to flexible solar cells, and factors affecting the cell efficiency are reviewed. Further efficiency improvement options are discussed. Cell stability, challenges, solutions and future prospects of CIGS solar cells are outlined.