This article is a study of the life of Karl Hartenstein (1894-1952) and his contribution to world mission. Three contributions of Hartenstein to mission theology are outlined, focusing on Hartenstein's approach to (1) the theology of religions and the missio Dei (missio), (2) ecumenism (unio), and (3) eschatology and suffering (passio). In the first place, Hartenstein's contribution to the theology of religions and the development of the idea of missio Dei was considerable. Regarding the former, his understanding of religions began with Barth's rejection of religion as unbelief, but was later modified to take, like Kraemer, a more dialectical stance in that religion was viewed both as a human attempt at self-salvation and as the human quest for divine salvation. Regarding the latter, Hartenstein coined the term in 1934. The expression shifted the emphasis away from an activist, church-centered understanding of mission to one that saw mission primarily as the action of God. But, unlike later developments of this theology, his understanding of the relationship between the missio Dei and the missio ecclesiae was always one of a close relationship. Second, Hartenstein was a strong supporter of the ecumenical unity of the church. His participation in Amsterdam in 1948 and his efforts to rebuild fellowship with the European churches after World War II must be seen together with the rejection of German nationalism through his strong support of the Confessing Church. Third, for Hartenstein the salvation-historical understanding of biblical theology was the key element for understanding mission. "Mission with a focus on the end" provided not only a correct understanding of mission, motivation for mission, and readiness for suffering; it also clarified the relationship between the missio Dei and the missio ecclesiae.