Cyclin-dependent kinase 5 (Cdk5) plays a key role in proper development of the nervous system. To be activated, Cdk5 associates with regulatory subunits not related to cyclins, such as p35 (the regulatory subunit of Cdk5). In this article, we review some of the experimental evidence supporting a central role for the Cdk5/p35 kinase in neuronal migration and process formation.