This monograph reconsiders the question of speech isochrony, the regular recurrence of (stressed) syllables in time, from an empirical point of view. It proposes a methodology for discovering isochrony auditorily in speech and for verifying it instrumentally in the acoustic laboratory. In a small-scale study of an English conversational extract, the gestalt-like rhythmic structures which isochrony creates are shown to have a hierarchical organization. Then in a large-scale study of a corpus of British and American radio phone-in programs and family table conversations, the function of speech rhythm at turn transitions is investigated. It is argued that speech rhythm serves as a metric for the timing of turn transitions in casual English conversation.