All metazoans that reproduce sexually have the ability to form gametes. Both types of gametes, the egg and the sperm, arise from germ cells, undergo a unique program of differentiation, and are destined to unite. The outcome of their union, the zygote, is a cell possessing the tremendous potential to build a new individual that expresses, maintains, and propagates characteristics of the species.The focus of this review is the development of the egg in animals, beginning with the formation of primordial germ cells in embryos. This process, known as oogenesis, has long captivated the attention of developmental biologists who have come to realize that the zygote inherits from the egg not only genetic material but also its cytoplasm. This maternal cytoplasm supports to varying degrees the development of the early embryo and is the basis for the conclusion that embryogenesis actually begins during oogenesis. A wealth of experimental evidence shows that mechanisms for establishing the germline and carrying out oogenesis in evolutionarily distant animals follow certain common themes. Several conserved principles and the known molecular mechanisms behind them are discussed below.