This is a theme issue of "Philosophical Transactions B". Community genetics is a synthesis of community ecology and evolutionary biology. It examines how genetic variation within a species affects interactions among species to change ecological community structure and diversity. Evolutionary interactions among species are often considered in the context of coevolution, where evolution in one species results in reciprocal evolution in the other. Community genetics is concerned with interactions among multiple species that may not necessarily be coevolving, but where the evolutionary potential (genetic variation) and life histories of the different species affect the phenotype of other interacting species. This approach begins to enable us to understand how complex ecological communities change over time under different environmental pressures, and to make better-informed and more predictive decisions on the management, conservation and restoration of both natural and agricultural systems. In turn, this has wide ranging implications for food security and dealing with environmental change. Similarly, applying a community genetic outlook to host-parasite and host-symbiont interactions provides new insights into disease resistance and control, and allows a more sophisticated treatment of key topics such as the evolution of virulence. Community genetics therefore spans both pure and applied science, casting old questions in a new light and bringing new challenges into closer focus.