*What are the cellular and molecular mechanisms that lead to the development of distinct cell identities during development? *How does Hedgehog signaling work? *How is the correct number of each cell type determined? *What are the cellular behaviors that lead to the development of tissue form? Our lab is addressing these broad questions by studying the development of zebrafish skeletal muscle. We are working on muscle development because muscle is a very abundant and easily accessible tissue, and also because diseases of muscle development are debilitating and common childhood diseases. We work on zebrafish because they are readily accessible for experimental manipulations throughout development and because a genetic approach to studying development is feasible in this vertebrate. Muscle fibers in most vertebrates can be broadly classified as either slow or fast. Slow muscle fibers contract slowly and they fatigue slowly; in fish, slow muscle is used for steady cruising through the water. Fast muscle fibers contract fast and fatigue fast; they are used to power rapid escape responses, such as when a predator appears. The earliest developing muscle fibers have an intrinsic fiber type identity. However, the mechanism that establishes this early fiber type identity has been completely mysterious. We are using experimental embryology and genetics, two powerful approaches that can be combined in zebrafish, to understand how the identity of these two muscle fiber types is established, and what regulates the number of each cell type (zebrafish embryonic muscle fiber types).