As a part of its 125th anniversary, Science published a special issue containing 125 questions facing the scientific community over the coming decades. Motivation for the proposed symposium is based on one of these highlighted problems: What determines species diversity? The symposium will bring together mathematicians and biologists to highlight emerging research into this major question in ecology and evolution. Patterns of diversity can be generated by interactions among trophic levels, such as predator-prey interactions and host-parasite coevolution, or by interactions within a trophic level, such as variation in resource use among competitors. Furthermore, diversity can be studied at the community to the population level, and down to cellular, molecular, and genetic level. The proposed symposium will spotlight interdisciplinary efforts of mathematicians and microbial experimentalists across these levels. Traditionally, research into what determines patterns of diversity has been approached via ecological and evolutionary research independently; however, the mechanisms that generate and diversity clearly operate on both time scales. This is demonstrated, in part, by titles of symposia at the 2008 Annual Meeting: “The Evolutionary Ecology of Metacommunities”, “Evolution in a Community Context”, and “Evolution and Life History Theory”. Our proposed symposium will bring the expertise of mathematicians, along with ecological and evolutionary approaches, to the table. There has been a recent explosion of studies on microbial model systems, which have proven particularly powerful for addressing questions about what ecological and evolutionary mechanisms drive species diversity. There is also an increased awareness among microbial experimentalists that mathematical models can provide a crucial support for the interpretation and generalization of experimental results as well as provide motivation for new experiments. Furthermore, mathematical models of how diversity is both maintained and generated are now being tested, revised, and fully challenged by empirical data. The proposed symposium will be comprised of an interdisciplinary group of speakers that are studying mechanisms that generate and maintain diversity on a number of different levels, from gene regulation to community interactions, using both mathematical models and experimental microbial systems. Our proposed symposium emerges from a working group sponsored by the National Evolutionary Synthesis Center (NESCent) entitled “Mathematical Models, Microbes and Evolutionary Diversification”, which brings together experts in mathematics, theoretical evolutionary ecology and microbial experimental ecology, and evolution. The symposium will begin with a synthesis and perspective plenary talk by Michael Doebeli, University of British Columbia, on this emerging approach to understanding mechanisms generating and maintaining diversity.