It is imperative that the method for evaluating and expressing uncertainty in measurement should be uniform among scientists and engineers so that measurements performed in different scientific and technological fields can be easily compared. This article attempts to familiarize scientists and engineers with general concepts of uncertainty in measurement that are based on the ISO Guide of Uncertainty in Measurement (GUM). This uncertainty consists of several components that may be grouped into two categories, A and B. These categories do not correspond simply to “random” and “systematic” uncertainties that have been used previously. The article explains what category A is and what category B is and how to classify the several components into theses categories. The importance of the uncertainty budget table is also emphasized.