During the embryonic development of the mouse brain essentially all neuronal progenitor cells migrate from their place of birth in the ventricular zone towards their target areas where they settle and differentiate. Two modes of migration are known: radial and tangential neuronal cell migration. Tangentially migrating neurons travel long distances parallel to the pial surface, whereas radially migrating neurons travel along processes of radial glia. The aim of this work was to clarify the mechanisms of tangential neuronal migration in the embryonic mouse. This study focussed on the posterior extramural migration stream (PES). A YT-PhPax6-tTA transgene mouse line was used to selectively label all Pax6 positive cells of this migration stream with GFP (green fluorescent protein). To investigate properties of migrating cells, transplantations, collagen cocultures and overexpression studies were performed. In addition, expression studies were used to identify genes, which may influence rhombencephalic cell migrations. My results demonstrate that neuronal precursors in the PES migrate autonomousely and not along preexisting fibers. Cells are attracted by the floor plate. Transplantation experiments show that the attractive signal in the floor plate is distributed homogenously along the rostro-caudal axis. The responsiveness to the attractive signal is lost once migrating neurons have crossed the midline. Collagen coculture experiments and expression studies suggest that the attractive signal in the floor plate is at least partly mediated by Netrin-1 and its receptors. Overexpression of the Netrin-1 receptor Unc5H3 in migrating PES cells resulted in apoptosis providing further evidence that this receptor acts as “dependence receptor”. Alongside the receptor is important for the correct migration of the cells from the pial surface into the parenchym to recognise their targets. In addition the proteins EphA3, EphA7 and EphrinB1 were identified to possibly regulate the tangential migrations in the rhombencephalon. Accessorily a direct accord of the expression patterns of Netrin1 and its receptors DCC, neogenin and Unc5H1-4 was carried out. The results suggest a possible role of these proteins not only for tangential migration in the rhombencephalon but also for tangential and radial migration in the telencephalon. Taken together, these studies provide for the first time insight to general mechanisms underlying tangential brain stem cell migrations. In addition, potential candidates were identified mediating guidance cues of tangentially migrating PES neurons.