 In the 1980s and 1990s, epistemic logicians focused on the logical properties of systems containing groups of knowers and later still on the epistemic features of so-called “multi-modal” contexts. Since the 1990s work in  dynamic epistemic logic  has extended traditional epistemic logic by modeling the dynamic process of knowledge acquisition and belief revision.  In the past two decades, epistemic logic has come to comprise a broad set of formal approaches to the interdisciplinary study of knowledge and belief.