This article presents an overview of the recent developments and requirements in radiotherapy dosimetry, with particular emphasis on the development of optical fibre dosemeters for radiotherapy applications, focusing particularly on in vivo applications. Optical fibres offer considerable advantages over conventional techniques for radiotherapy dosimetry, owing to their small size, immunity to electromagnetic interferences, and suitability for remote monitoring and multiplexing. The small dimensions of optical fibre-based dosemeters, together with being lightweight and flexible, mean that they are minimally invasive and thus particularly suited to in vivo dosimetry. This means that the sensor can be placed directly inside a patient, for example, for brachytherapy treatments, the optical fibres could be placed in the tumour itself or into nearby critical tissues requiring monitoring, via the same applicators or needles used for the treatment delivery thereby providing real-time dosimetric information. The article outlines the principal sensor design systems along with some of the main strengths and weaknesses associated with the development of these techniques. The successful demonstration of these sensors in a range of different clinical environments is also presented.