This article evaluates six mobility-related device classifications for their ability to support assistive technology outcomes research. Our evaluation considered classifications that had been created for various purposes, including those created to support third-party reimbursement decisions, consumer education and safety, and research. Classifications were excluded if their scope was limited to a single mobility device domain. The six classifications were analyzed according to a common framework: (1) purpose, (2) completeness, (3) granularity, and (4) research applications. Although each classification addresses three principal mobility device domains (ambulation aids, manual wheelchairs, and powered mobility devices), the analysis revealed a range of detail with which each domain is described. Some classifications were hampered by their use of unclear idiosyncratic terminology, whereas others conflated multiple device features within device categories. The analysis suggests that existing classifications do not fully meet the needs of assistive technology outcomes researchers. Creation of a common taxonomy of mobility devices is needed to serve the needs of the assistive technology outcomes research field.