This paper describes the conceptual basis for the development of a new clinical evaluation instrument, the Tufts Assessment of Motor Performance (TAMP). The TAMP is a 32-item, diagnosis-independent, criterion-referenced test that samples physical performance items in the areas of mobility, activities of daily living and physical aspects of communication. The administrative and scoring criteria of the TAMP are presented, and the multiple measurement dimensions are described. The documentation of patient status and progress, as described in the functional and performance profiles, is outlined. The paper also reports initial interobserver reliability on the intraitem tasks and the summary indexes of the two profiles. Forty individuals (20 adults and 20 children) with neurologic and musculoskeletal disorders comprised the reliability sample. Kappa and intraclass correlations were used to estimate the reliability of three independent raters on individual tasks and aggregate scores, respectively. Task reliability for the assistance and approach measurement dimensions were generally higher than for the more qualitative pattern and proficiency dimensions. Yet over 90% of all the tasks had acceptable reliability, while all the summary indexes had high interobserver reliability. Determination of interobserver reliability data is the initial phase of defining the most appropriate and technically valuable items, and will serve as a basis for item revision and reduction to enhance the clinical utility of the test.