Embryonic development has fascinated scientists and philosophers from ancient culture to the present day. This chapter explores embryonic development. In attempts to unravel the mysteries of embryonic development, a few specific animal species have been used repeatedly, most often for straightforward practical reasons such as whether the animals of interest were readily available. As time has gone by, other species have gained popularity for embryo research, again initially for reasons of availability but also for simplicity and short generation (or gestation) times. These include the roundworm (Caenorhabditis elegans), the fruit fly (Drosophila melanogaster), the zebrafish (Dania rerio), the African clawed frog (Xenopus laevis), the chicken (Gallus domesticus), and the house mouse (Mus musculus). It has become increasingly clear that the key features of embryonic development have remained for the most part unaltered by evolution. While animals show obvious differences in appearance, the majority of their genes are well preserved, demonstrating roughly similar structure and function. However, despite years of dedicated research, much still remains to be discovered on the formation of gametes (the sex cells), fertilization, and the subsequent development of the embryo. Both ethical concerns and limited availability have always restricted research using human embryos so that most knowledge on human embryonic development stems from extrapolating results obtained using laboratory mice. While many parallels exist between embryonic development of mice and men, significant differences are also evident.