The generally accepted approach to evaluating and expressing uncertainties in measurements undertaken by testing and calibration laboratories is given in The Guide to the expression of Uncertainty in Measurement, first published in 1993 by ISO, Geneva. That document, frequently referred to as The Guide or the GUM, presents a comprehensive study of these principles which, if followed, will ensure that measurement uncertainty is calculated and stated in a consistent manner in all situations. It is, however, a complex document and it has been found necessary to supplement it with simplified guidance for specific fields of measurement.This publication is one such guidance document that follows on from Stephanie Bell’s A Beginner’s Guide to Uncertainty of Measurement, developing treatment of the subject to a level intended for testing laboratories. It presents principles and guidance for the estimation of measurement uncertainty which are applicable to most fields of testing but is not intended for calibration laboratories, which normally need to have a greater depth of knowledge of these principles than is presented here. The principles and guidance given here are consistent with The Guide and with the requirements of ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories where they relate to estimating and reporting uncertainty of measurements.