Reliability evaluation of circuits operating in harsh environments is essential to prevent functional failures and potential catastrophic events. To achieve this, it is important to have a solid understanding of different radiation environments and their effects on matter. This chapter aims to provide the necessary foundations to comprehend the dynamics of radiation environments, including space and Earth’s atmosphere, as well as particle accelerators. Additionally, the chapter explores the prominent effects resulting from the interaction of energetic particles with matter. This knowledge is crucial for developing strategies to mitigate the impact of radiation on circuit reliability and ensure the proper functioning of electronic systems in challenging environments.