We are fascinated by beneficial plant-microbe interactions, particularly those between nitrogen-fixing bacteria and plants. Much of our work is in the legume-rhizobia system, utilizing natural genetic variation in both partners to ask questions about adaptation and conflict. We are also working in grass-associative nitrogen fixation systems to understand the functioning of interrelationships between plants and their rhizosphere communities and the evolutionary stability and molecular basis of these interactions. Finally, we use mathematical models to explore the conceptual underpinnings of mutualisms with the goal of integrating models with empirical data to understand fundamental processes acting at molecular, organismal, population/community, and ecosystem scales.