This book approaches the concept of 'boundary', central in linguistic theory, and the related notion of 'phase' from the perspective of the interaction between syntax and its interfaces. A primary notion is that phases are the appropriate domains to explain most interface linguistic phenomena and that the study of (narrow) interfaces helps to understand conditions on the internal structure of the Language Faculty. The first part of this volume is dedicated to introducing the notion of boundary, cycle and phase, and also the current debates regarding internal interfaces, in particular, the syntax-phonology, syntax-semantics, syntax-discourse, syntax-morphology and syntax-lexicon interfaces, in order to show how the notion of boundary/phase is related to (or even determines) most of their characteristics. The four sections of the second part deal with (morpho)phonology/ syntax and the role or boundaries/phases; the syntax-discourse and syntax-semantics interface; and the lexicon-syntax interface, while the notion of boundary/phase cross-cuts the main topics addressed.