This book provides a detailed and up to date review of the framework of phases (Chomsky 2000 and subsequent work). It explores the interaction between the narrow syntactic computation and the external systems from a minimalist perspective. As has sometimes been noted, <i>Phase Theory</i> is the current way to study the cyclic nature of the system, and 'phases' are therefore the natural locality hallmark, being directly relevant for phenomena such as binding, agreement, movement, islands, reconstruction, or stress assignment. This work discusses the different approaches to phases that have been proposed in the recent literature, arguing in favor of the thesis that the points of cyclic transfer are to be related to uninterpretable morphology (the Φ-features on the heads C and v*). This take on phases is adopted in order to investigate raising structures, binding, subjunctive dependents, and object shift (word order) in Romance languages, as well as the nature of islands.