This thesis examines the missiological writings of Orlando E. Costas (1943-1987), particularly The Church and Its Mission: A Shattering Critique from the Third World (1974); Theology of the Crossroads in Contemporary Latin America (1976); Christ Outside the Gate (1982); and Liberating News: A Theology of Contextual Evangelization (1989). From the early 1970s until his death in 1987 he wrote over 130 articles and 12 books in both Spanish and English that addressed key missiological concerns. A careful reading of a selection of Costas's texts oriented around a hymn, a gospel song, a psalm, and a poem provides the shape of this thesis. This thesis argues that Costas formulated a Third World evangelical missiology. Chapter one investigates what Costas's autobiographical material expressed about his positions on conversion, Protestant evangelicalism, missiology, and those living on the ‘periphery' of life. Chapter two recognises his commitment to the peoples of Latin America and the Caribbean in particular and the Third World in general. Chapter three explores Costas's analysis of the Latin American Protestant Church in a revolutionary situation in the continent and chapter four examines his survey and critical appraisal of Latin American liberation theology. Chapter five recognizes the pastoral shape of Costas's missiology. Chapter six explores his critical interaction with two more conservative evangelical missiological positions, the Church Growth Movement and Peter Beyerhaus and the Frankfurt Declaration, and chapter seven surveys the discussion within the international evangelical community regarding the relationship between evangelism and social responsibility. Chapter eight examines Costas's Liberating News as an expression of Third World evangelical missiology. Chapter nine considers the theological issue of penal substitutionary atonement and his missiology. The thesis concludes with an appraisal of the issues and contributions of Costas's Third World evangelical missiology to current missiological discussion.