We study neuronal development during embryogenesis and in the adult brain. We focus on three regions in the forebrain: the cortex, the basal forebrain and the olfactory bulb. We examine the genetic basis of neuronal subtype specification, migration, differentiation and circuit integration. Using genetic tools we fate-map precursor cells in the embryonic brain and identify their neuronal progeny. We tag and purify neuronal subtypes in order to identify genes involved in their development. We manipulate gene function using loss- and gain-of-function approaches. We are also developing genetic tools to trace neuronal connections in vivo and manipulate neuronal activity in order to examine participation in neuronal circuits. These tools will also provide us with an unprecedented ability to interfere with neuronal function in free moving animals.