Embracing more than 5,000 genera, distributed in 425 families and 46 orders, Malcolm C. McKenna and Susan K. Bell’s Classification of Mammals is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus. Since George Gaylord Simpson’s 1945 classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization. McKenna inherited the project from Simpson and, with Bell, has constructed a completely updated hierarchical system that reflects the genealogy of Mammalia.