Oral mucositis (OM) is a serious consequence of some chemotherapy and radiotherapy regimens. A number of reliable instruments are available to assess OM, but none are universally accepted. A unique collaboration of multi-disciplinary experts from Europe was formed to make recommendations on OM assessment, based on a systematic literature review and the experts' experience. The main recommendations are listed. There should be a comprehensive baseline assessment. OM should be frequently assessed using a standardised instrument, or a combination of instruments. Physical, functional and subjective changes should be measured. Subjective measures should be assessed prior to any physical examination. The use of pain scoring, in particular patient self-reporting, should form part of any OM assessment. Any assessment instrument should be validated, easy to use and comfortable for the patient. Training of, and monitoring in, the use of the instrument is vital to successful monitoring of OM.