The objective of this book is to give well-grounded methods for estimation of the uncertainly of measurement results. Starting from the basics of metrology, the book studies this subject in detail, from theoretical analysis all the way to concrete practical recommendations in areas ranging from single measurements in industry, trade, etc. to multiple measurements in experimental sciences. The book derives these recommendations by systematic development of the measurement accuracy theory. An important aspect of this book is that it develops the theory from the strong practical perspective, by giving priority to the physical essence of the problems and paying special attention to properties of measuring instruments and their influence on the uncertainty of measurements. The book also presents basics on characterization, standardization and calibration of measuring instruments as well as methods for calculating limits of errors of measuring instruments. All recommendations are illustrated by detailed examples from measurements of both electrical and mechanical quantities.