During CNS development, intrinsic and extrinsic signals guide the migration of neurons to form laminae, compartments in which neurons differentiate. Defects in neuronal migration are known to cause brain malformations in humans, but whether defects in glial cells, which provide environmental cues for neuronal migration, cause similar problems is less clear. Yue and colleagues now provide the most compelling evidence to date that Bergmann glia are important for neuronal migration in the mouse cerebellum (see p. 3281). They show first that deleting the tumour suppressor gene Pten in both neurons and glia leads to severe lamination defects. Then, by specifically deleting Pten in Bergmann glial cells, they reveal that the premature differentiation of Bergmann glial cells caused by PTEN loss produces severe defects in cerebellar neuron migration and laminar formation. Thus, they conclude, Bergmann glia provide crucial, developmental stage-dependent extrinsic cues during cerebellar development.