This thesis is a theoretical investigation of the status of "phases" within the phase theory with a particular emphasis on several constructions in Japanese and other languages that involve Case. I argue for a contextual approach to phases, where Case determines the relevant context for phasehood. In chapter 2 I argue that phases are determined by Case-valuation based on an analysis of the scope puzzle with Nominative/Accusative conversion in Japanese. Armed with a phase-based analysis QR, I argue that the scope puzzle is best accounted for if phases are determined by Case-valuation. The crucial observation in this chapter is that v Ps with a full set of argument structure whose head is not involved in Case-valuation do not constitute phases. In chapter 3 I extend the Case/phase hypothesis proposed in chapter 2 to various phenomena in Japanese and other languages and explore the possibility that all major projections (CPs, NPs, PPs, APs) work as phases only when their heads are involved in Case-valuation. I discuss (i) A-movement out of CPs, which I argue is possible only in certain contexts, (ii) extraction out of Traditional Noun Phrases, (ii) extraction out of PPs, and (iv) Government Transparency Corollary (GTC) effects (Baker 1988). In chapter 4 I discuss NP-ellipsis in Japanese. I propose a phasal reinterpretation of Saito and Murasugi's (1990)/Lobeck's (1990) claim that functional categories allow ellipsis of their complements only under Spec-Head agreement. This chapter also provides evidence that Japanese lacks D, as proposed by Fukui (1986, 1988) and Boskovic (2008, 2010a, 2010b), among others. I develop an analysis in which a head that bears a case-particle (K) is a phase head, which licenses ellipsis of its NP-complement under certain conditions. In chapter 5 I discuss restructuring infinitives in Japanese and show that there is a general ban on adjunction to complements of lexical restructuring verbs, which is best explained by an interaction of contextual emergence of phases and Case feature checking. It is also shown that this ban regulates adverb insertion, adjective insertion, and quantifier raising. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]