Four of the chapters in the fourth volume of Advances in Heterocyclic Chemistry deal with topics which have never been reviewed before. The first simple isothiazole was studied by Dr. R. Slack in 1956, and he and Dr. K. R. H. Wooldridge review in their article the considerable progress which has been made since then. Similarly the discovery of the widespread occurrence of covalent hydration has been achieved by the Canberra School, and we now have authoritative reviews on the qualitative (Drs. A. Albert and W. L. F. Armarego) and quantitative (Dr. D. D. Perrin) aspects of covalent hydration. Following the realization of the importance of aryne intermediates in benzenoid chemistry, work has been done on hetarynes, and this field is now reviewed by Professor H. J. den Hertog and Dr. H. C. van der Plas who have themselves contributed much to this subject. The volume also includes a review of oxazolone chemistry by Dr. R. Filler and a survey of the reactions of azines with nucleophiles by Drs. R. G. Shepherd and J. L. Fedrick which complements the previous review by Professor Illuminati in Volume 3 of this series. The last chapter forms a very significant contribution to heterocyclic chemistry in correlating an enormous amount of data, scattered throughout the literature, on this important subject.