▪ Abstract Theoretical calculations, based on both the chemical and isotopic composition of sedimentary rocks, indicate that atmospheric O₂ has varied appreciably over Phanerozoic time, with a notable excursion during the Permo-Carboniferous reaching levels as high as 35% O₂. This agrees with measurements of the carbon isotopic composition of fossil plants together with experiments and calculations on the effect of O₂ on photosynthetic carbon isotope fractionation. The principal cause of the excursion was the rise of large vascular land plants and the consequent increased global burial of organic matter. Higher levels of O₂ are consistent with the presence of Permo-Carboniferous giant insects, and preliminary experiments indicate that insect body size can increase with elevated O₂. Higher O₂ also may have caused more extensive, possibly catastrophic, wildfires. To check this, realistic burning experiments are needed to examine the effects of elevated O₂ on fire behavior.