The complexity of modern radiotherapy requires a comprehensive quality assurance programme, including in vivo dosimetry. In this paper, the use of the detector systems most often used for in vivo dosimetry [diodes, thermoluminescence detectors, metal oxide field effect transistors and electronic portal imaging devices (EPIDs)] will be summarised. Although point detectors are useful for the verification of conventional 3-D conformal radiotherapy, the use of 2-D detector systems, such as EPIDs, is required for the verification of more complicated techniques, including intensity-modulated radiotherapy.