Metrics is often defined as a discipline that concerns itself with the study of meters. In this volume the term is used in a broader sense that more or less coincides with the traditional notion of 'versification'. Understood this way, metrics is an eminently complex object that displays variation over time and in space, that concerns forms of a great variety and with different statuses (meters, rhymes, stanzas, prescribed forms, syllabification rules, nursery rhymes, slogans, musical text setting, ablaut reduplication etc.), and that as a cultural manifestation is performed in a variety of ways (sung, chanted, spoken, read) that can have direct consequences on how it is structured. This profusion of forms is thought to correspond, at the level of perception, to a limited number of cognitive mechanisms that allow us to perceive and to represent regularly iterating forms. This volume proposes a relatively coherent overall vision by distinguishing four main families of metrical forms, each clearly independent of the others and amenable to separate typologies.