Instrument reliability is defined and related to other measures of durability such as MTTF and failure rate. The need for reliability in modern instrumentation is discussed. A quantitative analysis is given of both maintained and nonmaintained instruments. Various measures for improving an instrument's reliability and availability are described, two of which, redundancy and maintenance, are treated in more detail. Illustrations of specific forms of redundancy are taken from the domain of analogue instrumentation. Preventive and corrective maintenance and their effectiveness are discussed.