Gamma rays are electromagnetic rays which are produced due to radioactive decay of atomic nuclei. Among all the EM waves, gamma rays possess the highest energy and hence the shortest wavelength. Lightning strikes, solar flares, cosmic rays and terrestrial gamma ray-flashes are some of the natural sources of gamma rays. Artificial sources of gamma rays include neutral pion decay, nuclear fusion and high energy physics experiments. Decay of Cobalt-60, Nickel-60 and Platinum-192 are used extensively to study gamma rays. They have a very high penetrating power and find extensive applications in the mining industry, pulp and paper industry, food industry, etc. PET scan and gamma-knife surgery are some of the techniques which employ gamma rays to diagnose and cure cancer. They are also used to alter the properties of semi-precious stones. This book traces the progress of this field and highlights some of its key concepts and applications. It explores all the important aspects of this field in the present day scenario. A number of latest researches have been included in this book to keep the readers up-to-date with the global concepts in this area of study.