Electricity production is currently responsible for a large share of global Greenhouse Gas (GHG), NOx and SO2 emissions, and their related environmental impacts. This study provides a critical review of the status of research on life cycle assessment (LCA) of electricity generation. NREL [1-6] recently suggested a harmonization of GHG emissions of single technologies: these studies certainly broaden the level of understanding of the variables that might lead to different results when performing LCA, but focusing on GHG may not lead towards environmental sustainability. Additionally, the present study focuses on the comparability between different technologies, identifying and quantifying the possible mistakes that can occur when comparing two technologies whose environmental assessments have been performed with conflicting assumptions. Nine different power generation technologies were examined: hard coal, lignite, natural gas, oil, nuclear, biomass, hydroelectric, solar photovoltaic and wind. More than 150 published studies were selected and analyzed to investigate whether "typical" GHG, NOx and SO2 emission factors for each technology could be identified. For a better overview of the sources of emissions, those were divided among three life cycle phases: fuel provision, operation of the plant and infrastructure. It was possible to estimate typical emission factors for all technologies except for biomass, where methodological and technical aspects result in very variable outcomes. Within these ranges, we identified direct emissions at the plant as the main contributor to the total GHG emissions for fossil fuels, with thermal efficiency being the most determining parameter. Nevertheless, with high thermal efficiency fuel provision becomes increasingly important (e.g. natural gas combined cycle). Different results were found concerning NOx and SO2. Concerning renewable energy sources, infrastructures were identified as the main contributor to all emissions. However, when considering renewables the emissions often covered a wide interval even within the same technology; this was mainly due to geographical factors and date and type of data used. We therefore suggest not to limit studies to GHG, and, to ensure comparability between studies, to transparently report emission factors for electricity production stating clearly the functional unit of the study, the efficiency for fossil technologies and the temporal and geographical scope for renewables.