This book is intended for the student familiar with the basics of axiomatic set theory, including an introduction to Goedel's theory of constructibility. It presents a thorough analysis of the first two approximations to the set-theoretic universe, given by the universes L and L[0#]. Goedel's constructible universe L provides the setting in which the most thorough understanding of set theory can be achieved, through use of the fine structure theory. The text begins with a streamlined treatment of the fine structure of L, using the notion of S* formula. It follows the technique of forcing with sets or classes, establishing basic facts about the preservation of ZFC and cofinalities. The model L[0#] then arises naturally as a way to select the "relevant" forcing extensions of L. The author shows that forcing, normally a tool for establishing relative consistency results, now becomes a powerful technique for analysing the set-theotic universe. He develops this theme by using class forcing to solve the Genericity, 12-Singleton and Admissibility Spectrum problems of Jensen and Solovay. The book's further applications of class forcing to genericity, admissibility, descriptive set theory and set-theoretic definability are sure to be of interest to a wide community of set theorists.