The origin of optical methods for fluid flow investigations appears to be nontraceable. This is no matter for surprise. After all seeing provides the most direct and common way for humans to learn about their environment. But at the same time some of the most sophisticated methods for doing measurements in fluids are also based on light and often laser light. A very large amount of material has been published in this area over the last two decades. Why then another publication? Well, the field is still in a state of rapid development. It is characterised by the use of results and methods developed within very different areas like optical physics, spectroscopy, communication systems, electronics and computer science, mechanical engineering, chemical engineering and, of course, fluid dynamics. We are not aware of a book containing both introductory and more advanced material that covers the same material as presented here. The book is the result of a compilation and expansion of material presented at a summer school on Optical Diagnosticsfor Flow Processes,held at RiS0 National Laboratory and the Technical University of Denmark in September 1993. The aim of the course was to provide a solid background for understanding, evaluating, and using modem optical diagnostic methods, addressing Ph. D. students and researchers active in areas of fluid flow research. The disciplines represented by the participants ranged from atmospheric fluid dynamics to biomedicine.