This paper reviews the principle features of human placenta structure and development, and it highlights areas of needed future research. The complexity of placental histology is discussed and comparison of the human placenta with other mammalian taxa is made. A more widespread appreciation of the value of studying this challenging organ and a more profound understanding of the regulation of the placental transport functions is advocated. Simple, routine examination of the placenta should be resurrected. Clin. Anat. 11:194–205, 1998. © 1998 Wiley-Liss, Inc.