Communities are important for contemporary life, but they are difficult to define and measure. In this talk, I will explore four ways that communities are often defined: as a population, as a place, as a network cluster, and as a balanced network. In each case, I consider the advantages and disadvantages of the approach. I then take a closer look at how each of these approaches can help us better understand communities in US politics, where traditional approaches define communities by political parties (i.e. as a population), but recent work that defines communities by collaboration and opposition (i.e. as a balanced network) reveals subtle new patterns.