 Günther Ludwig (1918–2007) was a German physicist mainly known for his work on the foundations of quantum theory. In Ludwig (1970, 1985, 1987), he published an axiomatic account of quantum mechanics, which was based on the statistical interpretation of quantum theory. As a prerequisite for this work he found it necessary to ask “What is a physical theory?” and developed a general concept of a theory on the first 80 pages of his (1970). Later this general theory was expanded into the book Ludwig (1978). A recent elaboration of Ludwig’s program can be found in Schröter (1996).