Genetic mosaics can be used to gain insight into the cell specificity of gene function. How Caenorhabditis elegans mosaics are typically generated is reviewed, and several examples with relevance to developmental studies are mentioned. One example is mpk-1, which encodes a member of the Ras-MAP-kinase pathway. mpk-1 mosaics have been a means of studying the distinct cells that require the gene for distinct fates during development. The gene bre-5 is used as an example of the usefulness of mosaic analysis for non-developmental studies. Potential problems with mosaic analysis are discussed, and the power of combining mosaic analysis with cell- or tissue-specific promoters is mentioned.