Research in physical medicine and rehabilitation. XII. Measurement tools with application to brain injury. There are basic principles and techniques of measurement that are relevant across biomedical disciplines. The purpose of this article is to explain some of the most important of these for medical rehabilitation, to illustrate how to use them to choose assessment instruments and to describe the nature of measurement in medical rehabilitation by examples in brain injury rehabilitation. Reliability is basic to any scientific measure. Validity, the ultimate criterion, is closely associated with the purpose of the measure. Content validity, criterion validity and construct validity are explained. Sensitivity to rehabilitative interventions and significance in patients' real lives (ecological validity) are emphasized. Measures of functional outcomes (disability) may show improvement after rehabilitation even when impairment measures do not. An extensive but selected list of measures of coma, global status, disabilities, communicative and cognitive impairments, and handicaps is presented, and their main uses are illustrated. Examples illustrate how to choose measures to study comprehensive program-level outcomes, to study learning-based interventions and to develop a general purpose database. Although there are many measures of activities of daily living and mobility, little published evidence of reliability and validity could be found even for some well-known scales. Ecologically valid and sensitive outcome measures are especially needed. Studies of the clinical utility of measures were also scarce. Many of these gaps can be spanned by clinical researchers with limited resources. Physical medicine and rehabilitation will benefit from formal studies of the reliabilities and validities of both its old and its new measurement instruments and by increased sophistication in choice of measures. 