This book investigates the concept of phase, aiming at a structural definition of the domains that are assumed as the syntactic loci for interface interpretation: vP, CP and DP. The 16 contributions address crucial issues of syntactic research within the Minimalist Program, namely A) How is the set of minimally necessary syntactic operations to be characterised, 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 investigation focuses on a variety of languages, combining the analysis of empirical data with the theoretical insights of the last years.