The problem of the composition of cosmic dust grains has stubbornly defied solution for over half a century. A succession of models have been proposed and their properties worked out for comparison with an ever-expanding body of relevant observational data. The authors began their pioneering work in this field in the 1960s by challenging the then popular ice grain theory. Most controversially they later hypothesised that condensed organic matter in the galaxy is of biological origin, thus linking an old astronomical problem with the question of the origin of terrestrial life. In this book the authors develop the theory of Cosmic Grains on a broad front starting logically from basic mathematical and astronomical premises. The reader is guided through a historical progression of ideas on the nature of grains, leading ultimately to the authors' own point of view, which shows through a clear predictive sequence the important role of complex organic material in the interstellar grains.