Pain and opioid use disorder (OUD) are inextricably linked, as the former can be a risk factor for the development of the latter, and over a third of persons with OUD suffer concomitant chronic pain. Assessing pain among people with OUD is challenging, be-cause ongoing opioid use brings changes in pain responses and most pain assess-ment tools have not been validated for this population. In this narrative review, we discuss the fundamentals of pain assessment for populations with OUD. First, we de-scribe biological, psychological, and social aspects of the pain experience among people with OUD, as well as how opioid-related phenomena and healthcare dispari-ties in this population may contribute to the pain experience. Second, we review meth-ods to assess pain including: (1) traditional self-reported methods, such visual analog scales, and structured questionnaires; (2) behavioral observations and physiological indicators; (3) and laboratory-based approaches, such as functional brain imaging, electroencephalography, and quantitative sensory testing. These methods are consid-ered from a perspective that encompasses both pain and OUD. Finally, we discuss strategies for improving pain assessment in persons with OUD and implications for future research, including educational strategies for multidisciplinary teams. Substan-tial gaps persist in our knowledge, particularly regarding the applicability of current pain assessment methods to persons with OUD, as well as the generalizability of the existing results from adjacent populations. As research linking pain and OUD evolves, considering the needs of diverse populations with complex psychosocial back-grounds, we will be better equipped to reduce these gaps.