Single case designs (SCDs) represent an excellent approach for developing and testing treatments, and for generating general knowledge of treatment mechanisms and outcomes. Their ability to generate knowledge, however, will depend on their methods being reliable, valid, clearly described, and replicable. This scoping review aims to map assessment methods in SCD studies of psychological treatments for chronic pain. The particular aims were to review the specific measures used, their modes of administration, their adequacy as measures, and opportunities for improvement. PubMed, PsycINFO, CINAHL and OpenGrey were searched for SCD studies of psychological treatments for chronic pain in adults. Two reviewers independently assessed eligibility, with input from two additional reviewers, and then extracted relevant data from the 55 included studies. The most common outcome domains were "pain", "physical functioning" and "psychological processes." Non-behavioral measures were more common than behavioral or physiological measures. Measures were often standardized. Measures in each domain were highly varied, and administration information was lacking. Evidence for adequacy of measures was based on psychometric analyses of aggregated group data for non-behavioral measures and agreement for behavioral and physiological measures, but was frequently not reported. Non-behavioral idiographic measures were often inadequate in that adapted measures were often used but validation was reported for original versions. Outcome domains did not correspond with currently available guidelines for measures employed in clinical trials in pain. A new set of guidelines, that is more suited to psychological treatments and to SCDs, is needed. Researchers should consider following an idiographic framework by using individualized measures more often. When measures are individually applied, it is recommended that their adequacy is not based on nomothetic psychometric approaches. Rather, behavioral assessment principles should be employed. Overall, adequacy reporting, usage of SCD consistent terminology, and information on how measures are administered need improvement.