This book contains basic and essential facts on measurement. Starting from a presentation of the concepts of physical quantity, the value of a physical quantity and unit of measurement, the authors discuss measurement as a process that has the object of determining a value of a quantity. Measurands are treated as stochastic variables, and a theoretical model for the result of measurement as an estimate of the value of the measured quantity is presented. The reliability of the results of measurement is discussed thoroughly; in particular, uncertainty of measurement and its estimation, taking into account other quantities that influence the result. In this connection, an account is given of relevant pars of mathematical statistics and the propagation of uncertainty of measurement. Separate sections deal with comparison and presentation of the results of measurement. Practical use of the described methods is illustrated by numerous examples. The terminology used, and the approach to uncertainty of measurement, are fully in line with internationally accepted recommendations.