There have been many interesting developments in the field of nuclear radiation detectors, especially in those using semiconduct­ ing materials. The purpose of this book is to present a survey of the developments in semiconductor detectors along with discus­ sions about gas counters and scintillation counters. These discus­ sions are directed to detector users, usually scientists and technicians in different fields such as chemistry, geology, bio­ chemistry, and medicine. The operation of these detectors is discussed in terms of basic properties, such as efficiency, energy resolution, and resolving time, which are defmed in the first chapter. Differences among these detectors in terms of these properties are pointed out. Chapter 2, on interaction of radiations with matter, discusses how different radiations lose energies in matter and how differences in their behavior in matter affect the design and operation of detectors. Although emphasis is placed on fundamentals throughout the book, the reader is also made aware of the new developments in the field of radiation quite often detection. The author has taught a course in radioisotopes for several years for science, engineering, medical, and dental students. The emphasis on topics varied from time to time to satisfy the varying interests of the students. However, the contents of this book formed the core of the course. About ten selected experiments on detectors were done along with this course (a list of these vii Preface viii experiments may be supplied on request).