The end-Permian (c. 252 Ma) and end-Triassic (c. 201 Ma) mass-extinction events are commonly linked to the emplacement of the large igneous provinces of the Siberia Traps and Central Atlantic Magmatic Province, respectively. Accordingly, scenarios for both extinctions are increasingly convergent and cross-fertilization of ideas has become important. Here, we present a synthesis of extinction scenarios based on a critical assessment of the available palaeontological, sedimentological, geochemical and geophysical evidence. How similar were the extinction events, what gaps exist in our understanding and how can a comparison of the events enhance our understanding of each event individually? Our focus is on the most important proximate kill mechanisms including: climate change and atmospheric pollution; increased soil erosion, weathering and runoff; forest dieback and the spread of pathogens; and ocean temperature changes, anoxia and acidification. There is substantial evidence to suggest that very similar kill mechanisms acted upon late Permian as well as Late Triassic ecosystems, strengthening the hypothesis that the ultimate causes of the mass-extinction events were similar.