Bacterial communities are enormously complex, with hundreds of interacting species in every drop of pond water. These communities are also ecologically and economically important: for example, they cycle nutrients to higher organisms and detoxify soil. Understanding how these complex communities impact these “ecosystem functions” is an important goal in microbial ecology research, but the research is constrained by the complexity of the communities. We will describe experiments using simplified communities to understand how interactions among bacteria change over ecological and evolutionary timescales, and will explain how we will extend the experiments to more complex communities.