The development of a multicellular organism from a single fertilized cell is a complex process that requires extensive cell division, growth, differentiation, and regional specification. Following the fertilization of an oocyte by a spermatozoon, cleavage divisions rapidly produce blastomeres. The blastomeres continue to divide and organize into the mammalian blastocyst, which contains an inner cell mass composed of embryonic stem cells. Signal transduction molecules and transcription factors promote the proliferation and differentiation of pluripotent embryonic stem cells into progenitor cells that will give rise to all cell types in the mature organism. During gastrulation, orchestrated cell migration results in germ layer formation (ectoderm, mesoderm, and endoderm), which produces regional specialization. Morphogens form gradients of environmental cues that regulate embryonic pattern formation and cell fate decisions, while adhesion molecules and proteases control cell migration and the molding of tissues into their characteristic forms. During organogenesis, specialized cues refine the formation and function of organ systems. For example, axon guidance molecules regulate nervous system development and growth factors and cytokines are responsible for controlling vasculogenesis, angiogenesis, and hematopoiesis. In addition, many of the molecular and cellular processes that govern embryonic development may be recapitulated in adult tissues during disease progression and regeneration.