The mammalian brain is composed of a vast (and unknown) number of specialized cell types whose complex interactions underlie behavior. To better appreciate these cellular specializations — and the genes that cells of diverse types use to perform their jobs — we used Drop-seq to analyze 690,000 individual cells from nine different regions of the adult mouse brain. We identified 1.45 billion RNA transcripts among these cells, then used the cells' patterns of RNA expression to classify them into transcriptionally distinct groups of cells (clusters and subclusters). Most of these clusters and subclusters correspond to discrete cell types; some to dynamic cell states; and many, we expect, to aspects of cell biology that remain to be discovered.