As the popularity of oil industry is decreasing and people are investing in finding sustainable solutions, the interest in renewable energy has been increasing. Especially the amount of offshore wind farms has been growing a lot during the past years. This is a study about how the offshore wind farms are constructed in Finland and what kinds of challenges are faced during the construction. This study is based on information from articles, project descriptions and interviews with offshore wind energy professionals. The thesis consists of four areas: general information of offshore wind farms and their substructures, the descriptions of construction processes of two existing offshore wind farms in Finland, the challenges and conclusions.