This technical guidance document is intended for scientists and practitioners who work with freshwaters in the field and in the laboratory but who do not have specialist knowledge of quality assurance. It provides an introduction to the key concepts and approaches that can be used in Quality Assurance and Quality Control. Without appropriate and adequate quality assurance measures at each step, from taking a water sample to storing and sharing the data, the outputs of the monitoring activities may not reflect, as fully as they should, the true quality condition of the water body. Although the main focus is on freshwaters, the general principles outlined are applicable to most types of aquatic monitoring including drinking water, wastewater, and industrial effluents.