Collapsin response mediator protein (CRMP) was originally identified as a molecule involved in semaphorin3A signaling. CRMPs are now known to consist of five homologous cytosolic proteins, CRMP1-5. All of them are phosphorylated and highly expressed in the developing and adult nervous system. In vitro experiments have clearly demonstrated that CRMPs play important roles in neuronal development and maturation through the regulation of their phosphorylation. Several recent knockout mice studies have revealed in vivo roles of CRMPs in neuronal migration, neuronal network formation, synapse formation, synaptic plasticity, and neuronal diseases. Dynamic spatiotemporal regulation of phosphorylation status of CRMPs is involved in many aspects of neuronal development.