Book excerpt: From the refinement of general methodology, to new insights of synchronic and diachronic universals, to studies of specific phenomena, this collection demonstrates the crucial role that language data play in the evolution of useful, accurate linguistic theories. Issues addressed include the determination of meaning in typological studies; a refined understanding of diachronic processes by including intentional, social, statistical, and level-determined phenomena; the reconsideration of categories such as sentence, evidential or adposition, and structures such as compounds or polysynthesis; the tension between formal simplicity and functional clarity; the inclusion of unusual systems in theoretical debates; and fresh approaches to Chinese classifiers, possession in Oceanic languages, and English aspect. This is a careful selection of papers presented at the International Symposium on Linguistic Diversity and Language Theories in Boulder, Colorado. The purpose of the Symposium was to confront fundamental issues in language structure and change with the rich variation of forms and functions observed across languages.