Recent genetic studies in Drosophila have provided important insights into the pathways determining the formation and diversification of body wall muscles. These pathways control a progressive subdivision of the mesoderm, ultimately leading to the specification of individual cells, the muscle founders, which are endowed with genetic programs capable of generating distinct muscle fibers. A network of activities of transcriptional regulators, signaling pathways, and lineage genes is beginning to emerge which controls successive steps of this muscle patterning and differentiation process.