The first complete genome sequence of a multicellular organism, Caenorhabditis elegans, has been determined recently. Several more will follow soon, among them the genome sequences of Drosophila melanogaster and the human. With these achievements, the stage is set for the next phase of "large-scale" biology, the study of the function of genes and the interactions between their protein products. The aim of the Ernst Schering Research Foundation Workshop 29 was to discuss the contribution of research on model organisms which are particularly suited for genetic and biological research, to the understanding of gen­ eral principles of biology as well as the basis for human diseases. This area of research has not only unveiled the basic molecular mechanisms of development of higher organisms, but has also made major contri­ butions to the elucidation of complex signal transduction pathways that play crucial roles both in ontogeny and human diseases. Establishing the complete molecular anatomy of an organism is a demanding but rather well-defined task. However, compared to the technical and scientific challenges of large-scale sequencing and the bioinformatics analysis of complete genome data, the assignment of function to the genes of multicellular organisms may still seem like a fishing expedition, with no universal road to success. One approach is based on large-scale technologies. The use of mi­ croarrays for multiple gene expression analysis is one element in this effort, providing important information on the cell- or tissue-specific activation of genes over time.