When semiconductor devices operate in a radiation environment (ex: space), they undergo severe degradation. The degradation behaviour is a complex process and is dependent not only on the nature of the device but also on the radiation characteristics viz., dose, dose rate, species and the energy of radiation. The study of the effect of radiation on semiconductor devices is important both from the academic as well as technological point of view. Academically it is very important to have an understanding of the physical mechanism of the damage process and technologically it is important to assess the device performance when they need to be operated in the radiation environment. This book involves the study of radiation induced effects in Bipolar Junction Transistors planned for space applications. The devices exposed to electromagnetic radiation, 8 MeV electrons and 24 MeV protons are characterized before and after irradiation to understand the radiation induced degradation mechanism. Apart from discrete devices, heavy ion induced effects on a few space borne VLSI devices have also been carried out and results are discussed. In addition to this, some simulation results are discussed.