The direct conversion of sunlight into electricity - photovoltaics (PV) - has emerged as a strongly growing market during the past years. Today, more than 80% of the world market is supplied by solar modules based on mono- or polycrystalline silicon wafers. Thin-film solar cells promise significantly lower costs for photovoltaic energy conversion, and thus will probably dominate the PV-market in the future. Consequently, the production of thin film solar cells will lead to key technologies of the 21 st century. This article addresses the three most advanced types of thin-film cells describing the status of these technologies at the laboratory level, in pilot production, and in first production lines. The challenges along the way from laboratory developments towards mass production are discussed, focusing on the central role of the vacuum-based technologies applied for thin film deposition.