ISO/IEC Guide 98-1:2009(E) provides a brief introduction to the 'Guide to the expression of uncertainty in measurement' (GUM) in order to indicate the relevance of that fundamental guide and promote its use. It also outlines documents related to the GUM that are intended to extend the application of that guide to broader categories and fields of practical problems. ISO/IEC Guide 98-1:2009(E) addresses measurement science at a level that is suitable for those readers who have received training at least to the second year of a science- or engineering-based degree course containing some teaching of probability theory and statistics. It also considers various concepts used in measurement science. In particular, it covers the need to characterize the quality of a measurement through appropriate statements of measurement uncertainty. This introductory document also outlines the recent evolution of thinking regarding measurement uncertainty.