Kurt Grelling (1886–1942) was one of Hans Reichenbach's closest collaborators in Berlin. He can be regarded as the third man in the Berlin Group besides to Reichenbach und Dubislav. He tried to keep the group running even after Reichenbach and Dubislav had left the city. In the paper, the scientific development of Grelling during his Berlin years is discussed, focussing on his relationship to Hans Reichenbach and their overlapping scientific interests. Grelling's activities in support of scientific philosophy will be presented, and his final work as a collaborator of Paul Oppenheim in Brussels.