Ecologists have traditionally viewed the total species diversity within a set of communities as the product of the average diversity within a community (alpha) and the diversity among the communities (beta). This multiplicative concept of species diversity contrasts with the lesser known idea that α- and β-diversities sum to give the total diversity. This additive partitioning of species diversity is nearly as old as the multiplicative concept, yet ecologists are just now beginning to use additive partitioning to examine patterns of species diversity. In this review we discuss why additive partitioning remained “hidden” until just a few years ago. The rediscovery of additive partitioning has expanded the way in which ecologists define and measure β-diversity. Beta diversity is no longer relegated to describing change only along an environmental gradient. Through additive partitioning, β-diversity is explicitly an average amount of diversity just as is α-diversity. We believe that the additive partitioning of diversity into α and β components will continue to become more widely used because it allows for a direct comparison of α- and β-diversities. It also has particular relevance for testing ecological theory concerned with the determinants of species diversity at multiple spatial scales and potential applications in conservation biology.