The analysis of genetically mosaic organisms, which carry subsets of cells that are mutant for one or more genes, can reveal important information about a gene's role in development. In the first of a series of educational Primer articles on this topic, Yochem and Herman (see p. 4761) describe how genetic mosaics are generated, identified and analysed in the nematode Caenorhabditis elegans. As they discuss, genetic mosaics can provide a way around the problem of embryonic lethality that is caused by some genetic mutations. They also allow researchers to ask key questions about how a gene functions in development, such as whether a gene acts cell autonomously (when its loss effects only the cells that express it) or non-cell autonomously(when its loss affects other, sometimes wild-type, cells). Genetic mosaic approaches to testing a gene's function have thus shed important light on developmental mechanisms in this and other organisms.