We have used time-lapse multiphoton microscopy to map the migration and settling pattern of GABAergic interneurons that originate in the ganglionic eminence of the ventral forebrain and incorporate into the neocortex of the cerebral hemispheres. Imaging of the surface of the cerebral hemispheres in both explant cultures and brains of living mouse embryos revealed that GABAergic interneurons migrating within the marginal zone originate from three different sources and migrate via distinct and independent streams. After reaching their areal destination, interneurons descend into the underlying cortex to assume positions with isochronically generated, radially derived neurons. The dynamics and pattern of cell migration in the marginal zone (see movies, available at www.jneurosci.org) suggest that the three populations of interneurons respond selectively to distinct local cues for directing their migration to the appropriate areas and layers of the neocortex. This approach opens a new avenue for study of normal and abnormal neuronal migration in their native environment and indicate that interneurons have specific programs for their areal and laminar deployment.