This review discusses examples of conserved cellular and molecular mechansims in development, including the pathway of signal transduction between the photoreceptors R8 and R7 in Drosophila, which is compared to vulval induction in Caenorhabditis elegans. The Wg pathway in Drosophila is compared, first, to the Wnt pathway in dorsal mesoderm specification in Xenopus: second, to the same pathway in sea urchins; third, to the equivalent in the mom cascade of C. elegans; and finally, to parts of the equivalent pathway in Dictyostelium discoideum. The conserved expression of some hox genes in vertebrate limb buds and in the heads or tails of several invertebrate and vertebrate embryos is also illustrated. Two further examples show the contrast between BMP4 and the proteins Noggin, Chordin, Follistatin and Gremlin. Another example concerns the binding of proteins to the 3'UTRs of several messages which inhibits their translation in Drosophila and C. elegans. The final example illustrates the selective transfer of transcription factors to nuclei in Drosophila and C. elegans.