We define a genetic species as a group of genetically compatible interbreeding natural populations that is genetically isolated from other such groups. This focus on genetic isolation rather than reproductive isolation distinguishes the Genetic Species Concept from the Biological Species Concept. Recognition of species that are genetically isolated (but not reproductively isolated) results in an enhanced understanding of biodiversity and the nature of speciation as well as speciation-based issues and evolution of mammals. We review criteria and methods for recognizing species of mammals and explore a theoretical scenario, the Bateson–Dobzhansky–Muller (BDM) model, for understanding and predicting genetic diversity and speciation in mammals. If the BDM model is operating in mammals, then genetically defined phylogroups would be predicted to occur within species defined by morphology, and phylogroups experiencing stabilizing selection will evolve genetic isolation without concomitant morphological diversification. Such species will be undetectable using classical skin and skull morphology (Morphological Species Concept). Using cytochrome-b data from sister species of mammals recognized by classical morphological studies, we estimated the number of phylogroups that exist within mammalian species and hypothesize that there will be >2,000 currently unrecognized species of mammals. Such an underestimation significantly affects conclusions on the nature of speciation in mammals, barriers associated with evolution of genetic isolation, estimates of biodiversity, design of conservation initiatives, zoonoses, and so on. A paradigm shift relative to this and other speciation-based issues will be needed. Data that will be effective in detecting these “morphologically cryptic genetic species” are genetic, especially DNA-sequence data. Application of the Genetic Species Concept uses genetic data from mitochondrial and nuclear genomes to identify species and species boundaries, the extent to which the integrity of the gene pool is protected, nature of hybridization (if present), and introgression. Genetic data are unique in understanding species because the use of genetic data 1) can quantify genetic divergence from different aspects of the genome (mitochondrial and nuclear genes, protein coding genes, regulatory genes, mobile DNA, microsatellites, chromosomal rearrangements, heterochromatin, etc.); 2) can provide divergence values that increase with time, providing an estimate of time since divergence; 3) can provide a population genetics perspective; 4) is less subject to convergence and parallelism relative to other sets of characters; 5) can identify monophyly, sister taxa, and presence or absence of introgression; and 6) can accurately identify hybrid individuals (kinship and source of hybrid individuals, F1s, backcrosses, direction of hybridization, and in concert with other data identify which hybrids are sterile or fertile). The proposed definition of the Genetic Species Concept is more compatible with a description of biodiversity of mammals than is “reproductively isolated species.” Genetic profiles of mammalian species will result in a genetic description of species and mammalian diversity, and such studies are being accelerated by technological advances that reduce cost and increase speed and efficiency of generating genetic data. We propose that this genetic revolution remain museum- and voucher specimen–based and that new names are based on a holotype (including associated tissues) deposited in an accredited museum.