The zero coding of referents or other clausal constituents is one of the most natural, communicatively and cognitively-transparent grammatical devices in human language. Together with its functional equivalent, obligatory pronominal agreement, zero is both extremely widespread cross-linguistically and highly frequent in natural text. In the domain of reference, zero represents, somewhat paradoxically, either anaphorically-governed high continuity or cataphorically-governed low topicality. And whether in conjoined/chained or syntactically-subordinate clauses, zero is extremely well-governed, at a level approaching 100% in natural text. The naturalness, cross-language ubiquity and well-governedness of zero have been largely obscured by an approach that, for 30-odd years, has considered it a typological exotica, the so-called "pro-drop" associated with a dubious "non-configurational" language type. The main aim of this book is to reaffirm the naturalness, universality and well-governedness of zero by studying it from four closely related perspectives: (i) cognitive and communicative function; (ii) natural-text distribution; (iii) cross-language typological distribution; and (iv) the diachronic rise of referent coding devices. The latter is particularly central to our understanding the functional interplay between zero anaphora, pronominal agreement and related referent-coding devices.