Ionizing radiations cannot be seen, felt or sensed by the human body in any way but excessive exposure to them may have adverse health effects. Radiation measuring instruments are needed in order to detect the presence of such radiations and avoid excessive exposure. The use of appropriate and efficient instruments enables exposures to be controlled and the doses received to be kept as low as reasonably achievable. This Manual explains the basic terminology associated with such measuring instruments and describes the principal types, their construction and typical applications in the workplace. It is important to ensure not only that monitoring is carried out where there is a potential radiation exposure but also that the monitoring instrument is appropriate to the task and that the user places correct interpretations on the results obtained. The Manual will be of most benefit if it forms part of more comprehensive training or is supplemented by the advice of a qualified expert in radiation protection. Some of the instrument tests and calibrations described in this Manual require the services of a qualified expert