Light-duty vehicles contribute considerably to global greenhouse gas emissions. Fuel cell vehicles (FCVs) may play a key role in mitigating these emissions without facing the same limitations in range and refueling time as battery electric vehicles (BEVs). In this study, we assess the environmental impacts and costs of a polymer electrolyte membrane fuel cell system (FCS) for use in light-duty FCVs and integrate these results into a comparative evaluation between FCVs, BEVs, and internal combustion engine vehicles (ICEVs). We conduct a detailed life cycle assessment (LCA) and cost assessment for the current state of the technology and two future scenarios for technological development. We compile a detailed and consistent inventory for the FCS by systematically disassembling and integrating information found in cost studies. For the vehicle-level comparison, we use models to ensure that vehicle size, performance, and fuel consumption are unbiased between vehicle types and consistent with the scenarios for technological development. Our results show that FCVs can decrease life cycle greenhouse gas emissions by 50 % compared to gasoline ICEVs if hydrogen is produced from renewable electricity, thus exhibiting similar emission levels as BEVs that are charged with the same electricity mix. If hydrogen is produced by natural gas reforming, FCVs are found to offer no greenhouse gas reductions, along with higher impacts in several other environmental impact categories. A major contributor to these impacts is the FCS, in particular the platinum in the catalyst and the carbon fiber in the hydrogen tank. The large amount of carbon fiber used in the tank was also the reason why we found that FCVs may not become fully cost competitive with ICEVs or BEVs, even when substantial technological development and mass production of all components is assumed. We conclude that FCVs only lead to lower greenhouse gas emissions than ICEVs if their fuel is sourced from renewable energy, as is the case with BEVs. FCVs are an attractive alternative to ICEVs in terms of vehicle performance criteria such as range and refueling time. However, the technological challenges associated with reducing other environmental impacts and costs of FCVs seem to be as large, if not larger, than those associated with the capacity and costs of batteries for BEVs-even when not taking into account the efforts required to build a hydrogen infrastructure network for road transportation.