In this book the reader will find the twilight process treated as an op­ tical phenomenon and as a tool for upper-atmosphere research. Our under­ standing of twilight has advanced considerably over the past 30 years. The acquisition mainly of descriptive material through rudimentary observing tech­ niques has now given way to purposeful quantitative research which exploits every facility of modern experimental technology. And the frankly specula­ tive arguments that had widely prevailed have been replaced by theory, de­ veloped in detail and tested experimentally, so that various aspects of the twilight phenomenon can now be treated from a rigorous mathematical stand­ point to yield much valuable data on the structure of the atmosphere. But al­ though these results have furnished a fairly comprehensive understanding of twilight, and have in fact established that the twilight method can be applied within certain limits for sounding the atmosphere, they remain scattered through papers in many different journals. It is therefore opportune now to collect the results and examine them from a unified viewpoint.The author has by no means attempted an exhaustive review of all twi­ light research —of the earlier work in particular. This book aims to outline the current status of the problem. Considerable space is devoted to ideas that the author has developed in recent years at the Atmospheric Optics Laboratory of the Institute of Atmospheric Physics, USSR Academy of Sciences. He has often reported this material verbally and much of it has been incorporated in published papers, but the results are here presented for the first time in com­ plete form, together with certain data from experimental studies of the twi­light sky conducted at the Atmospheric Optics Laboratory over the past few years. As is customary, omission of references to the literature indicates that the material is original.Translated from the Russian by Richard B. Rodman