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.