The Blue Brain Cell Atlas provides the densities and positions of all excitatory, inhibitory and neuromodulatory neurons, as well as astrocytes, oligodendrocytes and microglia in each of the brain regions defined in the Allen Mouse Brain Atlas. Blue Brain’s Cell Atlas was constructed by placing cells computed from whole brain Nissl and gene expression stains from the Allen Institute for Brain Science. Gene expression stains were used to decide the major type of cell. The API provided by the Allen Mouse Brain Atlas was used to access the data. Cell numbers and tissue distributions were validated against values from the literature where available. We found that the literature only contains specific values for 4% of the 737 brain regions. The atlas fills the gap of cell numbers for 96% of brain regions. Blue Brain found that experimental data cannot provide ground truth-values and therefore we developed a dynamic cell atlas that can integrate diverse datasets to converge towards ground truth-values, in principle for every cell-type in all brain regions. More detailed classification of cell-types can therefore be included and estimates for these can be iteratively improved, as new data is integrated. The user can also submit region and cell type specific densities and numbers obtained in new experiments for comparison and, to help with the convergence towards the ground truth. New data will be used to further improve both the next iteration of the Cell Atlas and its validation.