Integrins are transmembrane receptors that are known to interact with the extracellular matrix and to be required for migration, proliferation,differentiation and apoptosis. We have generated mice with a neural crest cell-specific deletion of the β1-integrin gene to analyse the role ofβ1-integrins in neural crest cell migration and differentiation. This targeted mutation caused death within a month of birth. The loss ofβ1-integrins from the embryo delayed the migration of Schwann cells along axons and induced multiple defects in spinal nerve arborisation and morphology. There was an almost complete absence of Schwann cells and sensory axon segregation and defective maturation in neuromuscular synaptogenesis. Thus, β1-integrins are important for the control of embryonic and postnatal peripheral nervous system development.