Contemporary conflicts over such issues as abortion, same-sex marriage, circumcision, and veiling highlight the need for renewed reflection on the justification of religious norms and authority. While abstract investigation of these questions is necessary, inquiry into them is not foreign to religious traditions. Philosophical engagement with these traditions of inquiry is both intellectually and practically advantageous. This does not demand, however, that these discussions be conducted within a discourse wholly internal to a particular religious tradition; dialogue between a religious tradition and philosophical reflection can be created that is mutually beneficial. To that end, this dissertation explores a central issue in philosophy of halakha (Jewish law): the relation between the justification of halakhic norms and halakhic-legal practice. A central component of philosophy of halakha is the project of ta'amei ha-mitzvot (the reasons for the commandments). Through such inquiry, Jewish thinkers attempt to demonstrate the rationality of Jewish religious practice by offering reasons for halakhic norms. At its best, it not only seeks to justify halakhic norms but also elicits sustained reflection on issues in moral philosophy, including justification and normativity. Still, there is a tendency among its practitioners to attempt to separate this project from halakhic-legal practice. Legal practice is thus isolated from philosophical reflection, and the reasons for the norms do not guide their application. Ta'amei ha-mitzvot therefore also provokes queries in legal philosophy concerning the relation between normative and legal justification. This study explores the relation between the justification of halakhic norms and halakhic-legal practice in modern Jewish thought by placing it into dialogue with both moral and legal philosophy. This occurs in two stages: First, the philosophies of halakha of three influential twentieth-century Jewish thinkers, Yeshayahu Leibowitz (1903-1994), Joseph Soloveitchik (1903-1993), and Eliezer Berkovits (1908-1992) are examined and critically assessed. It is shown that despite the denials of Leibowitz and Soloveitchik, all their accounts of the reasons for the commandments influence their approaches to halakhic-legal practice; they each combine a foundationalist approach to justification with skepticism about the practical normativity of reason; and none of them adequately grounds halakhic-legal authority. However, their skepticism is based on unduly constricted conceptions of reason and untenable alternative sources of normativity, such as will, metaphysics, or revelation. Second, through engagements with the work of Jürgen Habermas and Joseph Raz an alternative to their accounts of the justification of halakhic norms and authority is developed. This alternative is described as critical philosophy of halakha, for it does not attempt to justify halakhic norms or authority but articulates the rational constraints on, and practical consequences of, their justification. In terms of justification, this account is contextualist, that is, pragmatic and intersubjective, rather than foundationalist, and it is responsive to failures of justification. Correspondingly, it entails pluralism yet avoids moral and epistemic relativism. In terms of authority, this account is instrumentalist and thus mediates between normative and legal justification without reducing the latter to the former. Consequently, authority is circumscribed as opposed to total. Critical philosophy of halakha therefore represents a method whereby the modern religious believer may hold herself accountable to both her faith and other individuals.