The Bees of Colorado is a synopsis of over a century of work on Colorado bee species. It contains an annotated list of 946 valid extant bee species, 47 of which are new to Colorado including one species recorded for the first time from the United States. County level distributional data are listed with each species. We review the history of Colorado bee research and researchers. We discuss gaps in our current understanding of the Colorado bee fauna. Since Colorado bees are diverse not only in species, but also in biology, we include a summary of various bee life history traits, such as levels of sociality, nesting biology, and floral associations.