(Abridged and translated) Organic photochemistry may be divided into three parts: theory which is the province of the physical chemist; instrumentation which requires the skill of both physicist and engineer; and preparation which falls within the sphere of the organic chemist. At one time the same person could cover all three fields without too much difficulty, but this has now become virtually impossible because the disciplines involved have expanded in both breadth and depth; it is there­ fore timely to have a separate treatment of preparative organic photo­ chemistry. There appears to be no review of the main photochemical reactions which includes the advances made in recent years available to the organic chemist working in the preparative field. An exception is the excellent "Photochemical Reactions" by C. R. MASSON, V. BOEKELHEIDE and W. A. NoYES JR., published in 1956, which gives a brief review of the reactions which are important in preparative organic photochemistry. The present monograph on the other hand seeks to provide a detailed survey for the chemist; the author does not set out to discuss every photo­ chemical reaction in the field of organic chemistry but he does include in addition to those of current interest in the preparative field some which are likely to be of interest in the future and which result in single end-products of known composition. The photochemical synthesis of highly polymerized products falls outside the scope of the work.