In viviparous lizards and snakes, embryos develop inside the pregnant female and are sustained by placental organs. Our research focuses on functional morphology and evolution of these placentas. We use light and electron microscopy (TEM and SEM) to understand how structural features of the placentas facilitate provision of oxygen, water, and nutrients to developing fetuses during gestation. Our published research has drawn on two clades with generalized placentas that mainly function in gas exchange (thamnophine snakes and North American fence lizards [Sceloporus]) and three clades with placentas specialized for nutrient provision (South American skinks [Mabuya], Mediterranean skinks [Chalcides], and African skinks [Trachylepis]). One unexpected finding is that even generalized placentas show specializations for nutrient provision. Another is that some lizards have evolved placentas as complex as those of some mammals, with an invasive ovo-implantation that allows fetal contact with maternal blood vessels.