No general work on the organic derivatives of sulfur trioxide has appeared since the publication of C.M. Suter's basic treatise The Organic Chemistry of Sulfur, which covered the literature through 1941. This book represents an effort to overcome this need The author's approach has been selective, rather than all-inclusive, with the general objective of supplying background information of possible value to the laboratory chemist. Recent trends have been emphasized. Chief among these is the recognition that, although the conventional rules of aromatic orientation are followed under kinetically controlled conditions during sulfonation with sulfuric acid or oleum, these rules are not always followed under conditions that are thermodynamically controlled. This effect is made possible by the reversible nature of the sulfonation reaction, a characteristic which has enough other implications to warrant consideration in a sweparate chapter. Another effect noted increasingly in all aspects of sulfonate preparation, as well as in desulfonation, is the importance of steric requirements. Many examples are noted of the steric assistance or hindrance to be expected in reactions involving moieties as large as sulfur trioxide or sulfite ion. Emphasis is also placed upon recent studies involving the reactions of sulfur trioxide itself, and its various adducts with organic compounds. An effort is made in appropriate cases to bring together isolated facts into possibly meaningful form in such a way as to indicate trends or generalizations.