The thesis is an exploration and assessment of the missiological writings of Bishop Lesslie Newbigin following his return to Britain in 1974. It begins by setting out the context of his subsequent work as being one of cultural transition from modernity to postmodernity, and draws missiological implications from this evolution. Against this background, it is the original thrust of this thesis that the work of Michael Polanyi provides the indispensable hermeneutical key with which to unlock Newbigin's thinking, both in its philosophical and missiological dimensions. The contours of this indebtedness are explored in detail, and the implications for Newbigin's work in the context of both modernity and postmodernity are examined. Polanyi's thought is shown to contribute positive and constructive elements to Newbigin's work, enabling him to engage with the need for cultural renewal upon fresh epistemological grounds, to challenge modernity's preoccupation with 'reason' as the only grounds for certainty, and to recall the Church to a renewed confidence in the gospel by emphasising its fiduciary foundations. Moreover, Polanyi's influence is also shown to strengthen Newbigin"s credentials as a 'postmodern' missiologist. The thesis proceeds to argue that Polanyi's influence also provides the key to understanding Newbigin's philosophical and missiological shortcomings. Two of these are developed in detail. Firstly, the importation of Polanyi's liberal existential approach to epistemological method is shown to work against Newbigin's more robust revelational methodology. Secondly. Polanyi's influence is shown to contribute to Newbigin's ultimately confusing exposition of his programme of 'public' truth. The thesis critiques this element of Newbigin's programme and shows that it founders upon the importation of a secular Polanyian notion of 'dogma' into Newbigin's overtly theological and missiological framework. The thesis concludes by assessing Newbigin's contribution to cultural critique and mission and suggests lines of enquiry in the light of his work.