In last decades, many studies have been performed about renewable energy systems. In addition to studies using a single renewable energy source, hybrid structures have been extensively researched. In these studies, there are subjects such as increasing efficiency, reducing costs, meeting energy demands, MPPT research to make maximum use of energy, estimating using existing data, and robust controller designs. In this study, researches about hybrid renewable energy sources are reviewed and explained. The researched subjects and obtained results are given with tables and compared.