The cortex is built through an intricately choreographed process that involves the integration of many different cell types. The cortex begins as a layer of neuroepithelial cells. Microglia from the yolk sac arrive early in development and invade the developing cortical plate. Projection neurons are generated in an inside out fashion beginning at E12.5. Interneurons migrate to the cortex from the ventral forebrain. Oligodendrocyte precursors arrive in the cortex in three waves from the MGE, LGE/CGE and from within the cortex itself. Later in corticogenesis, progenitors initiate production of astrocytes. Abbreviations: IP, intermediate progenitor; NE, neuroepithelial cell; OL, oligodendrocyte; OPC, oligodendrocyte precursor cell; PV, parvalbumin; RG, radial glia; SST, somatostatin; 5HT3aR, serotonin receptor 3a.