This chapter is where Dr. Kollmann’s original manuscript starts. His introduction of the fundamental aerothermodynamic principles governing the design of centrifugal (radial) compressors is not in a form that one would find in standard textbooks, vintage or modern. Dr. Kollmann essentially lists the pertinent equations in the form of a ‘crib sheet’ that an experienced engineer would consult in his or her daily work. This is not surprising because, in all likelihood, when he was writing this manuscript (or at least this part of it) Dr. Kollmann did not have access to all his papers and books. In fact, the authors have a suspicion (a rather strong one from the cover page, as shown in the title page of the present book) that Dr. Kollmann wrote this section while he was actually in Allied custody or at ‘home supervision’.