The complete description of its nearly invariant cell lineage (1,2) and the growing availability of cloned genes and markers for the cell lineage make Caenorhabditis elegans particularly favorable for mosaic analysis, and the literature is rich in examples that prove the usefulness of this approach. Because genetic mosaic analysis in C. elegans has recently been reviewed by Herman (3) who developed many of the techniques (3–6), this review will be more concerned with recent technical advances rather than with an extensive background of the approach. First we shall present a brief summary of the principles of mosaic analysis as it is typically performed in C. elegans. This will be followed by a description of markers that indicate mosaicism and by a discussion of a hypothetical analysis of an essential gene.