Optically transparent zebrafish embryos provide an excellent vertebrate model system in which to reveal specific mRNA and protein expression patterns during development. Whole-mount preparations can be used to generate three-dimensional color or fluorescent readouts of the expression pattern of a given gene (or genes), matched with a bright-field image of all the tissues in the developing embryo. Whole-mount mRNA in situhybridization (WISH) has long been the method of choice for revealing gene expression patterns in zebrafish because this method depends only on being able to identify a relatively short region of nucleotide sequence unique for the gene of interest. In contrast, the scarcity of antibodies that are specific to or cross-react with zebrafish proteins has limited the widespread use of immunocytochemical applications, though this situation will improve in the future. The elucidation of the specific expression patterns of Wnt pathway genes in zebrafish has made a major contribution to our current understanding of their roles in vertebrate development.