With an understanding of male and female reproductive anatomy and endocrinology, we are ready to follow human reproduction and development from the formation of sperm and eggs, to fertilization, to subsequent development of the embryo and fetus, and finally to birth. Along the way, we consider how these processes vary in different organisms. Our understanding of these processes is in large part due to studies in model organisms, including the sea urchin (Strongylocentrotus purpuratus), roundworm (Caenorhabditis elegans), fruit fly (Drosophila melanogaster), African clawed frog (Xenopus laevis), and house mouse (Mus musculus). These organisms have particular reproductive and developmental features that make them amenable to laboratory study.