This book investigates the concept of phase, aiming at a structural definition of the three domains that are assumed as the syntactic loci for interface interpretation, namely vP, CP and DP. In particular, three basic issues are addressed, that represent major questions of syntactic research within the Minimalist Program in the last decade. A) How is the set of minimally necessary syntactic operations to be characterised (including questions about the exact nature of copy and merge, the status of remnant movement, the role of head movement in the grammar), B) How is the set of minimally necessary functional heads to be characterised that determine the built-up and the interpretation of syntactic objects and C) How do these syntactic operations and objects interact with principles and requirements that are thought to hold at the two interfaces. The concept of phase has also implications for the research on the functional make-up of syntactic objects, implying that functional projections not only apply in a (universally given) hierarchy but split up in various phases pertaining to the head they are related to. This volume provides major contributions to this ongoing discussion, investigating these issues in a variety of languages (Berber, Dutch, English, German, Modern Greek, Hebrew, Italian, Norwegian and West Flemish) and combining the analysis of empirical data with the theoretical insights of the last years.