The aim of this book Phases in the Theory of Grammar is to explain the phase theory in the Minimalist Program (Chomsky 1998, 1999, 2004, 2005). The notion of phase plays a crucial role in almost every aspect of the current theory from phonology to semantics across languages. This book brings together five research papers which investigate the most significant current issues on phases in the theory of grammar. In Chapter 2, Cyclicity and Phase are examined with notions of a phase, cyclicity, and cyclic Spell-Out in the Strong Minimalist Thesis. It is claimed that a phase is a special cyclic node which include CP and v*P, strong phase cycles, in contrast to vP, the weak phase cycle. The weak phase cycles of the passive, unaccusative and raising vP induce the effects of Phase Impenetrability Condition. It is argued that cyclic agreement solves the problem with these three weak phases through agreement between finite T and DP in an earlier phase, in contrast to the derivation of Spec-head agreement. Hence, it is concluded that the movements of both Spec-T and Spec-C in the successive cyclic movement have to be considered as one phenomenon of C, operating in parallel in a cyclic fashion. In Chapter 3, The Phasal Movement Hypothesis and Syntactic Freezing are investigated, supporting the phasal movement hypothesis. Clause-internal scrambling is shown to be EF-movement by claiming that EF-movement may be induced by EF of a non-phase head like T as well as by EF of a phase head like C, satisfying the six criteria of EF-movement. Criterion 4 of discourse effects for EF-movement is motivated in terms of the syntactic freezing phenomena, for which the D-Property Freezing Principle is motivated better than the Criterial Freezing Principle (Rizzi 2004b) and the Inactivation Freezing Principle (Chomsky 2005). Hence, discourse effects rather than scopal effects are shown to be the crucial property of EF-movement. In Chapter 4, Infinitives and EPP in terms of C-to-T Feature Inheritance are examined. It is analyzed whether EPP can be reformulated in terms of feature inheritance, utilizing two hypotheses: the hypothesis for the extended feature inheritance of the C-T category, and the split C and T hypothesis. The former includes the edge feature EPP as well as Agree and Tense features. In the latter, the finite Cs and Ts are distinguished from the infinitival Co and To. Hence, EPP is an EF of the C-T category and C-to-T feature inheritance is obligatory for EPP. T without C, a sole ToP, cannot have an EPP feature, leading to non-cyclic movement. Hence, English infinitives fall into three basic classes: sole ToP for raising, VP-ToP for ECM, and CoP-ToP for control and for-infinitives. In Chapter 5, The Edge-Feature-Driven EPP is dealt with, claiming that the phase-oriented EF motivates EPP-driven movement. EPP is linguistic properties that may be manifested in terms of the notion of EF. Recently Chomsky claimed that every operation is driven by phase heads (C, v, D) throughout the computational system. The interpretive complex of each phase is systematically correlated with the property of EPP across languages. For the EF-driven EPP, it is argued that the EPP can be satisfied by phase heads and the EPP realization by phase heads is the unmarked mode of the EPP effects. In Chapter 6, Phases and Words are investigated, claiming that the combination of Distributed Morphology with phase-based syntactic theory makes strong claims about the locality of phonological and semantic interpretation in words, as well as in larger syntactic structures. The new approach to derivational morphology lies in limiting the lexical properties to a single domain connected to the first category-determining head above the root and in equating category changing morphology with inflection. The tight connection between locality domains in morphophonology and in syntax is supported by identifying the unaccusative vP as a phase.