Thesis: Ph. D., Massachusetts Institute of Technology, Department of Brain and Cognitive Sciences, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 112-126).Human language is notable for its expressivity; syntax is powerful and allows for potentially unlimited new sentences. But even simple transitive sentences like "I broke the lamp" provide a sophisticated tool for communication, capture the basic building blocks of syntax and semantics that are widely agreed to be part of our linguistic capacity like agent or subject. With this relatively simple machinery, we are able to move a cognitive representation of an event from one person's head to another. How is this possible? In this dissertation, I examine both adult and child language to understand this capacity. Paper 1 examines the link between non-linguistic cognition and preschoolers' expectations about the meaning of novel verbs. We find that even though transitive verbs can refer to many event types, 3- and 4-year-olds are more likely to associate them with scenes with spatiotemporal features indicating causation. Papers 2 and 3 ask a second question: how do people organize language to facilitate communication? Paper 2 probes how adults order the basic elements (Subject, Verb, Object) in a task that appears to be independent of native language constraints, and tests whether the content of the message leads gesturers to reorganize their utterances. Paper 3 asks whether adults and children are aware that the relative informativity of arguments depends on context, and whether they can successfully make decisions in a novel communication task. By limiting the expression of transitive sentences to just two words (e.g. MONKEY EAT), we discover which elements people consider to be most informative. Both adults and children flexibly adjust their expectations about informative sentences according to which arguments are the most ambiguous in context. Together, these case studies help us understand how human language accomplishes its communicative goals, both in terms of the cognitive representations recruited for processing complex events in language, and the strategies used for expressing them. Whatever the formal nature of the representations involved in syntax and semantics, they must ultimately allow us to form predicates over nonlinguistic representations of the world, and they must support the kinds of pragmatic inferences that we know people can make.by Melissa Kline.Ph. D.