The processes of implantation and placentation have been studied in 105 tubal pregnancies. Successful placentation occurred only with plicomural or mural implantations and in these the early development of the placenta occurred in the tube in exactly the same way as it does in the uterus, although tubal placentation was usually membranaceous in nature, possibly because of an inadequate blood supply. Invasion of the placental bed by interstitial extravillous cytotrophoblast occurred in the tube in the same manner as it does in the uterus whereas extravillous trophoblast invaded tubal vessels in the same fashion as it does the spiral arteries in the uterus. Subsequent development of the placenta in the tube differed from that in the uterus in so far as villous invasion of the tubal tissues, with consequent development of the placenta in the subserosa or on the serosa, was common. The ability of the blastocyst to implant in the tube with subsequent normal placentation indicates that these processes are purely a function of trophoblastic tissue, with maternal tissues playing only a passive role.