The requirement for all potentially hazardous plant is to achieve high reliability of engineering systems by design. The process of reliability analysis is a fundamental part of the design process in the nuclear power industry. Such analysis recognizes that there is always some possibility of engineering equipment failing and therefore the ability of the plant to be reasonably tolerant of such failures is investigated. In this paper the methods and philosophy underlying reliability analysis are briefly explained with examples of qualitative techniques such as failure modes and effects analysis, and fault tree analysis. In addition some of the quantitative models of equipment reliability are discussed and the need for robust statistical techniques for data analysis explained.