The transition to digital technology has changed the nature of instrumentation and control (l&C) systems by enabling extensive interconnection of reprogrammable, functionally interdependent I&C systems. This development has made computer security a necessary element for consideration in I&C system design. The benefits and challenges of the various computer security methods and controls with their implementation in nuclear power plant I&C systems are discussed and described in this publication. The publication provides an overview of current knowledge, up to date good practices, experience, and benefits and challenges related to the application of computer security measures. The publication defines the key concepts for computer security for I&C systems at nuclear facilities, explains the risk informed approach to computer security and describes how computer security measures are applied throughout the l&C system life cycle. Situations where I&C systems are interconnected with enterprise management systems are also addressed. The three appendices present case studies with practical application examples.