Background: Observing and recording the signs and symptoms of oral mucositis are an important part of oral care, essential to the prevention and treatment of mucositis. Structured oral assessment enables a more informed and accurate identification of signs and symptoms and will enable early and individualized interventions. Objective: A United Kingdom-based mouth-care group conducted a systematic review of the published literature through to March 2004 and repeated in 2008. The goal of this review was to identify and evaluate the range of instruments used to assess oral mucositis to recommend in evidence-based guidelines the "best" instrument to use in the field of children's and young people's cancer care. Methods: Search sources included the Cochrane Library, MEDLINE, EMBASE, and CINAHL. Studies were selected using defined criteria and reviewed by 3 pairs of group members. Results: Fifty-four individual oral assessment instruments were identified with only 15 reporting evidence of reliability and validity testing. Only 3 articles reported on oral assessment exclusively in our population. Conclusions: The guidelines recommend only 1 assessment instrument, the Oral Assessment Guide, or adaptations of this instrument, to be used in clinical practice. Five factors influenced this recommendation: purpose of assessment, population, outcomes assessed, and quality of the instrument and ease of use. Implications for Practice: The Oral Assessment Guide has been consistently judged to be user-friendly and appropriate for everyday clinical practice with both adults and children, as well as a useful research tool.