Karl Kupfmuller (1897-1977) was a German engineer who began his professional career in the comparatively young telecommunications sector just after the First World War. During the 1920s, he worked for eight years at Siemens & Halske in Berlin. From 1928 onward, he held academic posts in electrical engineering, culminating as the chair of telecommunications in Darmstadt until 1963. While employed at Siemens & Halske, Kupfmuller carried out fundamental work on telegraphy and telephony, network theory, and electrical signal transmission. Like his U.S. contemporary Harry Nyquist, Kupfmuller derived fundamental results in information transmission and closed-loop modeling, including a stability criterion.