Book excerpt: Cells in the developing embryo depend on signals from the extracellular environment to help guide their differentiation. An important mediator in this process is the extracellular matrix – secreted macromolecules that interact to form large protein networks outside the cell. During development, the extracellular matrix serves to separate adjacent cell groups, participates in establishing morphogenic gradients, and, through its ability to interact directly will cell-surface receptors, provides developmental clocks and positional information. This volume discusses how the extracellular matrix influences fundamental developmental processes and how model systems can be used to elucidate ECM function. The topics addressed range from how ECM influences early development as well as repair processes in the adult that recapitulate developmental pathways.