The use of standardized tools for the assessment of competency is vital, given that informal assessments performed by physicians are idiosyncratic and unreliable . A number of instruments have been developed for this purpose, which are outlined in the following review. For the most part, these tools rest on a definition of competency put forward by Appelbaum and Roth [Appelbaum, P. S., & Roth, L. H. (1982). Competency to consent to research. A psychiatric overview. Archives of General Psychiatry, 39, 951-958], which incorporates various legal standards. Therefore, the present review will summarize current thinking on competence as well as the strengths, weaknesses, and psychometric properties of existing measures. As competency assessment will be more or less necessary depending on the patient population, another goal of the paper was to assemble the major findings pertaining to patient groups that have impairments in capacity. Although competency varies between and within diagnostic groups, empirical studies consistently demonstrate increased risk for some populations, and clinicians or researchers should be aware of this information. As such, this review may be useful as an initial source for interested researchers or clinicians.