Individuals with and acquired brain injury (ABI) often experience low motivation to engage in rehabilitation. However, measures of motivation and engagement in ABI populations are scarce. Currently, four such measures exist:​ the Brain Injury Rehabilitation Trust Motivation Questionnaire-Self (BMQ-S), the Motivation for Traumatic Brain Injury Rehabilitation Questionnaire (MOT-Q), the BMQ-Relative (BMQ-R), and the Rehabilitation Therapy Engagement Scale (RTES), but are without sufficient psychometric investigations and lack a theoretical framework. The purpose of this thesis was to evaluate the psychometric properties of the BMQ-S, MOT-Q, BMQ-R, and RTES in adults with an ABI, and to discuss how motivation theory can inform assessment of motivation in ABI. Thirty-nine adult ABI participants and 20 clinicians were recruited from an ABI rehabilitation program. Patient participants completed the BMQ-S, the MOT-Q, and self-rated measures of insight, apathy, depression, and anxiety. Clinician participants completed the BMQ-R, the RTES, and clinician-rated measures of insight and apathy. The MOT-Q and the BMQ-S had excellent internal consistency and test-retest reliability. The MOT-Q correlated with insight, while the BMQ-S correlated with apathy, depression, anxiety, and insight. The MOT-Q and the BMQ-S did not correlate with each other. The RTES and BMQ-R had excellent internal consistency and good inter-rater reliability. The RTES and the BMQ-R correlated with apathy and insight, and with each other. In light of the result that the BMQ-S and the MOT-Q did not correlate, it was determined that each measure may represent equally important but distinct aspects of motivation. By drawing upon Selfdetermination Theory, it was concluded that the BMQ-S and the MOT-Q may represent intrinsic and extrinsic motivation, respectively, and that utilizing both can provide a more comprehensive understanding of what factors are influencing a patient’s level of motivation to engage.