The present lectures describe the modern theory of critical phenomena, with emphasis on applications to real fluids. The aim is not to treat the subject exhaustively, but rather to explain the principal ideas, and to provide a guide to the vast literature on critical phenomena for the noninitiate. The original lectures consisted of three parts: (i) An account of static critical phenomena in fluids (theory and experiment), (ii) a brief introduction to the mathematics of the renormalization group, and (iii) a discussion of dynamic critical behavior. Only the first part is written up in any detail in the present notes, since there exist many recent reviews of renormalization group theory on both an elementary and an advanced level. In particular, the author has just completed a review article with B. I. Halperin on critical dynamics, to which the reader is referred for the contents of item (iii) above (Hohenberg and Halperin, 1977).