OBJECTIVES: Various assessment instruments are used for gathering data on different functional domains (i.e., mental), from various reporters (e.g., patient or observer). The objective of this study was to create an inventory of functional instruments used in patients with cognitive impairment and to categorize them using a taxonomy that combines several approaches: functional domain, data reporter, and performance versus opinion based. METHODS:  A literature search was conducted using the following: English language, any date, “performance based measure”, “performance measures”, “performance tests”, “cognitive function tests”, “functional measures”, “functional tests”, “cognitive measures”, “functional assessment”, “cognitive impairment”, “Alzheimer’s disease”. Three reviewers abstracted and evaluated retrieved studies to identify measure instruments and assign categories: 1. Addressing physical, mental, and/or social functioning; 2. Performance based (PerfO) or opinion based; and 3) Patient-reported (PRO), clinician-reported (ClinRO), or observer reported (ObsRO) outcome.  Discrepancies among reviewers were decided through consensus. RESULTS: The literature search led to the identification of 212 measures used in cognitive impairment.  Reviewers identified 134 as PerfOs. The remaining 78 opinion-based measures were categorized into PROs, ClinROs, and ObsROs. Overall, one, 17, and 152 measures assessed social, physical, and mental functioning, respectively.  Forty-two measures assessed multiple domains.  Within the 134 PerfOs, 13 were designed to assess physical, 109 mental, and one social functioning.  Eleven included multiple functional domains. CONCLUSIONS: When measuring function in patients with cognitive impairment, it is important to recognize the wide range of instruments available and their characteristics.  The taxonomy applied here, organizing functional-measure instruments used in cognitively impaired populations by various characteristics, is recommended as a starting point for characterizing tools. Researchers and clinicians can use such an approach to better understand the complexity of the measures as well as the strengths and limitations of these tools to guide them in selection and use, and interpretation of findings.