Water electrolysis is a promising technology for storing surplus energy from intermittent renewable energy sources in the form of hydrogen. The future investment costs of water electrolysis represent one key challenge for a hydrogen-based energy system. In this work, a literature review was conducted to evaluate the published data on investment costs and learning rates for PEM and alkaline electrolyzers from the 1990s until 2017 and the years beyond. The collected data are adjusted for inflation and specified in 2017 per kW-output using the higher heating value (HHV). R&D efforts have led to impressive cost reductions in the observed period, especially for PEM technology, while cost reductions for alkaline technology have also been decent. The overall spread of the cost estimations in the 1990s was in a range between 306 and 4748 (sic)(2017)/kW(HHV-output). Today's estimations for future investment costs (through 2030) for both technologies are narrowed towards values of 397 and 955 (sic)(2017)/kW(HHV-output). Higher automation, mass production, larger cell areas, market penetration and technology development will all have a further impact on the investment costs. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.