This publication describes a process for planning and conducting a project to implement digital instrumentation and control (I&C) systems in the modernization of a nuclear power plant (NPP). Many of the existing NPPs in the world are approaching, or have reached, the midpoint of their design life. At the same time, there have been tremendous advances in electronics, computers and networks. These new technologies have been incorporated into the digital I&C hardware and software currently available. Even though advanced digital I&C systems have been used extensively in many other industries, their use in the nuclear industry is still very limited. The complexity of digital I&C systems requires a comprehensive implementation plan to ensure that plant safety is maintained, and this publication presents the experience gained to date. It is intended to be of use to those involved in the design or implementation of such modernization projects.