Co-speech gestures are ubiquitous: when people speak, they almost always produce gestures. Gestures reflect content in the mind of the speaker, often under the radar and frequently using rich mental images that complement speech. What are gestures doing? Why do we use them? This book is the first to systematically explore the functions of gesture in speaking, thinking, and communicating - focusing on the variety of purposes served for the gesturer as well as for the viewer of gestures. Chapters in this edited volume present a range of diverse perspectives (including neural, cognitive, social, developmental and educational), consider gestural behavior in multiple contexts (conversation, narration, persuasion, intervention, and instruction), and utilize an array of methodological approaches (including both naturalistic and experimental). The book demonstrates that gesture influences how humans develop ideas, express and share those ideas to create community, and engineer innovative solutions to problems.