Genetic screens for recessive mutations continue to provide the basis for much of the modern work on Drosophila developmental genetics. However, many of the mutations isolated in these screens cause embryonic or early larval lethality. Studying the effects of such mutations on later developmental events is still possible, however, using genetic mosaic techniques, which limit losses or gains of genetic function to specific tissues and cells, and to selected stages of development. A variety of genetic mosaic techniques have been developed, and these have led to key insights into developmental processes in the fly. Variations on these techniques can also be used to screen for novel genes that are involved in non-embryonic patterning and growth.