These expository lectures contain an advanced technical account of a branch of mathematical analysis. In his own lucid and readable style the author begins with a comprehensive review of the methods of bounded operators in a Hilbert space. He then goes on to discuss a wide variety of applications including Fredholm theory and more specifically his own specialty of mathematical quantum theory. included also are an extensive and up-to-date list of references enabling the reader to delve more deeply into this topical subject.