The dissertation is a study of the concept of "mystery' in Second Temple Judaism, especially as this concept is expressed by the Hebrew and Aramaic word raz. It characterizes the raz-concept as one that arises out of Israel's prophetic and sapiential traditions, is an important feature of Jewish apocalyptic literature of the mid-late Second Temple period and becomes a central theological and cosmological reference point for the Yahad associated with the manuscript discoveries of Khirbet Qumran. The dissertation provides a comprehensive analysis of the semantic range of the term raz in the Qumran literature, and demonstrates that its verbal associations suggest a dynamic that includes the revelation, knowledge, and concealment of "mysteries' among the sect and its opponents. Especially in the Qumran sectarian literature, the raz-concept is employed in a fashion that draws upon prophetic, sapiential and priestly discourses as a way of authorizing the Yahad's claims to special, esoteric knowledge. As seen in compositions such as the Community Rule, Pesher Habakkuk, the Hodayot, the War Rule, the Mysteries texts, the Songs of the Sabbath Sacrifice and related works, the content of such knowledge included aspects of both the cosmological structure of the created order and the unfolding of historical (eschatological) time, and such knowledge was evidently guarded by the Yahad as necessary for guiding its communal rationale, structure and praxis.