Rudolf Haag (17 August 1922 – 5 January 2016) was a German theoretical physicist, who mainly dealt with fundamental questions of quantum field theory. He was one of the founders of the modern formulation of quantum field theory and he identified the formal structure in terms of the principle of locality and local observables. He also made important advances in the foundations of quantum statistical mechanics. Biography Rudolf Haag was born on 17 August 1922, in Tübingen, a university town in the middle of Baden-Württemberg. His family belonged to the cultured middle class. Haag's mother was the writer and politician Anna Haag. His father, Albert Haag, was a teacher of mathematics at a Gymnasium. After finishing high-school in 1939, he visited his sister in London shortly before the beginning of World War II. He was interned as an enemy alien and spent the war in a camp of German civilians in Manitoba. There he used his spare-time after the daily compulsory labour to study physics and mathematics as an autodidact. After the war, Haag returned to Germany and enrolled at the Technical University of Stuttgart in 1946, where he graduated as a physicist in 1948. In 1951, he received his doctorate at the University of Munich under the supervision of Fritz Bopp and became his assistant until 1956. In April 1953, he joined the CERN theoretical study group in Copenhagen directed by Niels Bohr. After a year, he returned to his assistant position in Munich and completed the German habilitation in 1954. From 1956 to 1957 he worked with Werner Heisenberg at the Max Planck Institute for Physics in Göttingen. From 1957 to 1959, he was a visiting professor at Princeton University and from 1959 to 1960 he worked at the University of Marseille. He became a professor of Physics at the University of Illinois Urbana-Champaign in 1960. In 1965, he and Res Jost founded the journal Communications in Mathematical Physics. Haag remained the first editor-in-chief until 1973. In 1966, he accepted the professorship position for theoretical physics at the University of Hamburg, where he stayed until he retired in 1987. After retirement, he worked on the concept of the quantum physical event. Haag developed an interest in music at an early age. He began learning the violin, but later preferred the piano, which he played almost every day. In 1948, Haag married Käthe Fues, with whom he had four