Verbs describe actions, and their subject, objects, and other complements refer to various participants in those actions. For a verb referring to a certain type of action, what linguistic principles determine which participant is to be referred to by its subject, which by its object, and so forth? Many previous approaches to this problem have employed a set of thematic roles, such as agent and patient, to classify kinds of participants. The alternative presented in this book relies on certain basic features of verb meaning, such as causation, and on a hierarchical classification of verb meanings, to develop a simple but widely applicable set of principles that account for the observed range of verb types in human languages.