The clinical evaluation of capacity to consent to treatment occurs in the medical setting and is based on legal foundations of informed consent and capacity. Clinical judgment is still the "gold standard" for capacity determination, although it can be unreliable. In the past 10 years the empirical basis for these assessments has been advanced considerably by the introduction of a number of instruments designed to assess capacity to consent to treatment. In this paper, we review studies, mostly with older adult populations, that consider the cognitive and non-cognitive correlates of consent capacity, rates of impaired capacity in various patient groups, the relation of instrument-based to clinician-based capacity assessment, and the inter-rater and test-retest reliability of consent capacity assessment. We also overview key research focusing on factors influencing, and procedural and processing variables involved in, medical decision-making. We conclude that these studies have yielded quite varied results, and promote no consensus regarding the reliability and validity of instrument-based consent capacity assessment. Overall, the results of these studies provide some guidance for clinicians, but, at present, practitioners should view these instruments as supplemental resources rather than benchmarks for assessment. However, this first generation of instruments provides a good foundation for future research, which should continue to systematically study aspects of reliability and validity, most especially construct validity, in well-defined patient populations.