Common ground is the information that the participants in a conversation treat as background information for the purposes of their interaction. We review two traditions of research on common ground. The formal tradition, consisting mainly of theoretical linguists and philosophers of language, has developed increasingly sophisticated formal models of common ground to generate predictions about an expanding range of empirical phenomena. Meanwhile, the psycholinguistic tradition has focused on a narrower range of phenomena while developing more realistic theories of the psychological mechanisms that allow us to select and represent common ground. After summarizing these two traditions, we consider several reasons why they should be reintegrated, and we argue that the best way to bring them back together would be to adopt a cognitive-pluralist approach, whereby language users have access to a variety of mechanisms for managing background information, which are more or less available and efficient depending on the communicative situation and the kind of information mentally represented as well as the cognitive demands of each mechanism.