This report replaces Technical Reports Series No. 239, Nuclear Power Plant Instrumentation and Control: A Guidebook (1984), in particular by changing the emphasis from guidance to summarizing operating experience and discussing new technologies. It provides an up to date overview of nuclear power plant instrumentation and control technology and the background against which such systems are implemented. It is directed to meet the needs of instrumentation and control engineers, but also of nuclear power plant designers and regulators. Contents: 1. General introduction; Part I. Requirements, constraints and recent issues; Part II. Design concepts; Part III. Recent developments in instrumentation and control; Part IV. Instrumentation and control in a new nuclear power plant; Part V. Examples of current instrumentation and control systems.