Elizabeth M. O'Dowd offers a new, discourse-functional account of the categories "preposition" and "particle" in English. She explains why certain words have membership in both categories, and solves many intriguing puzzles long associated with the syntax and semantics of these words. Based on linguistic data extracted from a series of actual conversations, O'Dowd provides new insights into how prepositions and particles are used, and how their meanings can change across different discourse contexts over time.