  This paper is an introduction to the von Neumann entropy in a historic
approach. Von Neumann's gedanken experiment is repeated, which led him to the
formula of thermodynamic entropy of a statistical operator. In the analysis of
his ideas we stress the role of superselection sectors and summarize von
Neumann's knowledge about quantum mechanical entropy. The final part of the
paper is devoted to important developments discovered long after von Neumann's
work. Subadditivity and the interpretation of the von Neumann entropy as
channel capacity are among those.
