Scanning electron microscopy of the placenta has a history of only twenty years. During that time, however, there have been dramatic advances in instrument technology coupled with the refinement of preparative techniques designed to reduce fixation artifacts to a minimum. As a result many of the early claims must be amended or suitably qualified, and this is one aim of the present review. Much new data on the internal structure of the placental villus is also presented. By means of the partial digestion technique it is now possible to describe the three dimensional configuration of the various components of the villous tree. This review will consider these sequentially, starting externally with the gross morphology of the villi and ending with replicas of the fetal vasculature produced by corrosion casting.