In this chapter I describe the complexity of roosts and their importance to understanding the ecology of phyllostomid bats and related noctilionoids, with an emphasis on the relationships between roosts, sociality, energetics, and species distribution. Detailed description of the roosts of phyllostomids is often neglected, due in part to the misconception that factors such as thermal stress are unimportant in tropical ecosystems. Although a number of systems for classifying roosts have been developed in the past, these schemes are in need of revision, as our knowledge about the ecology of bats has increased. I present a revised classification of roost types and identify gaps in our knowledge. In some respects this system of classification remains inevitably subjective, pending the accumulation of more detailed data. New directions for the characterization of roosts are suggested to allow an improved description of bat roosting ecology.