Basic and advanced principles of genetic analysis applied to the study of the best-understood eukaryotic model organisms including the nematode worm Caenorhabditis elegans, the zebrafish, and the laboratory mouse. We emphasize the use of genetic approaches to address problems in cell biology and development, such as the regulation of cell fate and tissue development. Much of the knowledge gained from these experimentally tractable organisms has proven broadly applicable, and the same principles of developmental genetic analysis underlie efforts to understand the development of humans.