**1. Introduction**

Preimplantation embryo development is characterized by a series of events after fertilization including the formation of the maternal and paternal pronuclei, followed by the formation of the zygote, which at ±20 hours after insemination starts undergoing mitotic divisions every 12–18 hours (cleavage stage), reaching the morula (compaction), and the blastocyst grows as the cells divide and the cavity expands, until it arrives at the uterus, where it "hatches" from the zona pellucida to implant into the endometrium. The main objective of blastocyst culture was to increase the success rate of in vitro fertilization (IVF) because of better embryo selection. Blastocyst culture has also been used as a tool to select the most viable embryos in a cohort with a consequent reduction in the number of embryos transferred and the corresponding reduction in the incidence of multiple gestations. This chapter will review about human blastocyst formation and development and discuss about the physiology, morphology, and gene expression of the blastocyst.
