This book fulfills the global need to evaluate measurement results along with the associated uncertainty. It includes details of uncertainty calculations for many physical parameters, and discusses probability distributions and their properties. Definitions of various terms are provided to help practicing metrologists grasp the subject. The book aids in establishing international standards for evaluating the quality of raw data obtained from various laboratories, facilitating the interpretation of results from different national metrology institutes in international inter-comparisons. For the routine calibration of instruments, a new idea for the use of pooled variance is introduced. Uncertainty calculations are explained for independent linear inputs, non-linear inputs, and correlated inputs. The merits and limitations of the Guide to the Expression of Uncertainty are also addressed.