This publication, which draws on the experiences of an IAEA coordinated research project (CRP) and on input from experts in the field, provides a comprehensive overview of the development of procedures for in vivo dosimetry in radiotherapy. It elaborates on the technology behind in vivo dosimetry and describes an initial set of measurements. Emphasis is given to patient dose studies, both evaluating the clinical value of in vivo dosimetry and comparing different in vivo dosimetry systems in a clinical setting. The findings of the CRP, which are summarized in this publication, will serve as a useful resource for hospital physicists seeking to establish an in vivo dosimetry programme in a radiotherapy centre and will help them in the selection of appropriate dosimetry systems.