The phenomenon of unaccusativity is a central focus for the study of the complex properties of verb classes. The Unaccusative Hypothesis, first formulated in 1978, claimed that there are two classes of intransitive verbs, the unaccusative (Jill arrived) and the unergative or agentive (Jill sings). The hypothesis has provided a rich context for debating whether syntactic behaviour is semantically or lexically determined, the consequence of syntactic context, or a combination of these factors. No consensus has been reached. This book combines contemporary approaches to the subject with several papers that have achieved a significant status even though formally unpublished.