The aim of this educational review, which is based upon expert opinion, is to describe to clinicians training in Physical and Rehabilitation Medicine and research students training to work in the field, the appropriate attributes and standards required for assessment and outcome measurement. 'What to assess' is discussed in the context of the conceptual framework provided by the International Classification of Functioning, Disability and Health, supplemented with quality of life as an additional construct. The reasons for making the assessment, and the context in which the assessment will be used, are then considered. Examples of recommendations of some international organizations regarding what and how to assess are presented. Suggestions are made about the selection of assessment tools, including examples from two diagnostic groups: stroke and rheumatoid arthritis. Finally, the basic psychometric standards required for any assessment tool, and additional requirements for outcome assessment, are explained.