Historically, the availability of genomic rather than phenotypic data has limited the scope of comparative genomics research. That was largely because natural history collections provided ample resources only for documenting physical traits, and also partly due to their evolutionary trajectories through time, as discussed by Michael Holmes and colleagues in “Natural History Collections as Windows on Evolutionary Processes.” Another factor was that the high cost of genome sequencing initially limited genomic data collections to the few highest-priority species—often with an emphasis on model organisms, for which research resources could be secured to support research that was relevant to human medicine. As the cost of sequencing has declined, partly resulting from NIH’s review of sequencing costs—for which the agency provides background information at its website (The Cost of Sequencing a Human Genome)—and as interest in trait mapping has extended from anatomical traits diagnosable in museum specimens to physiological and biochemical ones that require direct measurements from living organisms, it is increasingly the paucity of species-level phenotypic data sets that are the limiting factor. Current efforts to address these bottlenecks include the PanTHERIA database of traits variation across eutherian mammals, discussed by Kate Jones and colleagues in “PanTHERIA: A Species‐Level Database of Life History, Ecology, and Geography of Extant and Recently Extinct Mammals,” and the MaTrics database of mammalian traits, reported by Clara Stefen and colleagues in “Phenotyping in the Era of Genomics: MaTrics—A Digital Character Matrix to Document Mammalian Phenotypic Traits.”