This publication is intended to present a basic overview of instrumentation and control (I&C) systems in nuclear power plants (NPPs) and to serve as a reference guide on the subject. Furthermore, it provides an explanation of the significant role I&C systems have in maintaining and improving safety, plant performance and economic returns. Numerous publications have been prepared to address these issues; the present publication places those technical publications within the context of a global view of NPP I&C systems and their life cycles.