The mammary gland is an epidermal appendage that begins to form during embryogenesis, but whose development is only completed during pregnancy. Each mammary gland begins as a budlike invagination of the surface ectoderm, which then gives rise to a simple duct system by birth. Subsequent development occurs during sexual maturation and during pregnancy and lactation. In this review, we outline the distinct stages of embryonic mammary development and discuss the molecular pathways involved in the regulation of morphogenesis at each stage. We also discuss the potential relevance of embryonic breast development to the pathophysiology of breast cancer and highlight questions for future research.