Immunological studies of single proteins in a single cell type have been complemented in recent years by larger studies, enabled by emerging high-throughput technologies. This trend has recently been exemplified by the discovery of gene networks controlling regulatory and effector αβ T cell subset development and human hematopoiesis. The Immunological Genome Project (ImmGen) aims to decipher the gene networks underpinning mouse hematopoiesis. The first phase, completed in 2012, profiled the transcriptome of 249 immune cell types. We discuss the utilities of the datasets in high-resolution mapping of the hematopoietic system. The immune transcriptome compendium has revealed unsuspected cell lineage relations and the network reconstruction has identified novel regulatory factors of hematopoiesis.