Paul Tillich is one of the most important theologians of the twentieth century, and he produced a corpus of work that is considered canon for Western theology. While his influence is broad in and beyond the field of religious studies, his historic or ideological connection to radical theology has been hotly debated for decades. This introduction engages Tillich's work as a whole but focuses upon what elements of Tillich are specifically employed by radical theologians, but more importantly, makes the case that Tillich is, in fact, not only influential upon radical theology but rather that he should be counted among them as a proto-radical Christian thinker.