Since the 1950s, the incidence of twin gestation has doubled and the incidence of triplets has increased approximately sevenfold in the United States. Of extreme concern is the fact that many of these multiple pregnancies are iatrogenic: 35% of twin gestations and 77% of higher-order pregnancies are the result of some form of infertility therapy. Anything that can be done to reduce the number of these multiple pregnancies would benefit our patients and society. Great hope is placed on emerging blastocyst technology, which has the potential of achieving higher pregnancy rates per embryo transfer while reducing the risk of multiple pregnancy. We present the evolution of the blastocyst transfer concept and the technical aspects involved. The article also outlines the experience with blastocyst culture and transfer at Brigham and Women's Hospital, Boston, and describes identifiers for application of blastocyst transfer. The number of eight-cell embryos on day 3 is an independent marker for the selection of patients who would benefit from transfer on day 5. With no eight-cell embryos on day 3, 0% and 33% pregnancies resulted from day 5 vs. day 3 transfers, suggesting that these cases would not benefit from day 5 transfer. When at least one eight-cell embryo is available, there is no difference in ongoing pregnancy rates between day 5 and day 3 transfers, but there is a significant decrease in multiple gestations with day 5 transfers.