Astrocytes, one of the most common cell types in the brain, are essential for processes ranging from neural development through potassium homeostasis to synaptic plasticity. Surprisingly, the developmental origins of astrocytes in the neocortex are still controversial. To investigate the patterns of astrocyte development in the neocortex we examined cortical development in a transgenic mouse in which a random, sparse subset of neural progenitors undergoes CRE/lox recombination, permanently labeling their progeny. We demonstrate that neural progenitors in neocortex generate discrete columnar structures that contain both projection neurons and protoplasmic astrocytes. Ninety-five percent of developmental cortical columns labeled in our system contained both astrocytes and neurons. The astrocyte to neuron ratio of labeled cells in a developmental column was 1:7.4, similar to the overall ratio of 1:8.4 across the entire gray matter of the neocortex, indicating that column-associated astrocytes account for the majority of protoplasmic astrocytes in neocortex. Most of the labeled columns contained multiple clusters of several astrocytes. Dividing cells were found at the base of neuronal columns at the beginning of gliogenesis, and later within the cortical layers, suggesting a mechanism by which astrocytes could be distributed within a column. These data indicate that radial glia are the source of both neurons and astrocytes in the neocortex, and that these two cell types are generated in a spatially restricted manner during cortical development.