This volume explores and develops the framework of phases (so-called Phase Theory), first introduced in Chomsky (2000). The antecedents of such framework go back to the well-known notion of "cycle," which concerns broader notions, such as compositionality, locality, and economy conditions. Within generative grammar, this idea of the cycle took a concrete form in the fifties, with Chomsky, Halle, and Lukoff's (1955) pioneering work on stress, later on extended in Chomsky & Halle (1968), Halle & Vergnaud (1987), and further applied to morpho-phonology (Mascaro 1976 and Kiparski 1982),...