This volume examines the conflicting factors that shape the content and form of grammatical rules in language usage. Speakers and addressees need to contend with these rules when expressing themselves and when trying to comprehend messages. For example, there are on-going competitions between the speaker's interests and the addressee's needs, or between constraints imposed by grammar and those imposed by online processing. These competitions influence a wide variety of systems, including case marking, agreement and word order, politeness forms, lexical choices, and the position of relative clauses.Chapters in the book analyse grammar and usage in adult language as well as first and second language acquisition, and the motivations that drive historical change. Several of the chapters seek explanations for the competitions involved, based on earlier accounts including the Competition Model, Natural Morphology, the functional-typological tradition, and Optimality Theory. The book will be of interest to linguists from a wide variety of backgrounds, particularly those interested in psycholinguistics, historical linguistics, philosophy of language, and language acquisition, from advanced undergraduate level upwards.