Reptiles are important model systems for examining the effect of temperature during development on the phenotype of individuals after hatching or birth. To assess whether squamate embryos exhibit adaptive variation in thermal biology, we derived three parameters: an index of developmental rate (DRI), the upper thermal limit for successful incubation, and the lower thermal limit for development for 28 species of lizard and 12 species of snake. The associations between developmental parameters and climatic and life-history variables were examined using both conventional statistics and phylogenetically controlled analyses. Residual DRI (i.e. DRI corrected for stage at oviposition and hatchling mass) was strongly associated with phylogenetic relationship. By contrast, the upper limit for development was negatively related to the amount of precipitation during the warmest quarter of the year, and the lower thermal limit for development was positively related to temperature during the warmest quarter of the year and the activity body temperatures of adults. These latter observations indicate that embryonic thermal physiology is adapted to large-scale environmental patterns, and that global climate change will impact embryonic development directly through impacts on nest temperature per se, as well as indirectly through impacts on the ability of gravid females to select suitable nest sites. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.