Abstract: In addition to the rise in emphasis on the use of brief interventions, there's been a movement for substance abuse and other mental health professionals to implement empirically supported treatments. Although the empirically supported treatment movement is controversial, practice related organizations like the American Psychological Association's Divisions 12 and 17 (clinical psychology and counseling psychology) have developed guidelines or principles in regard to this issue. The empirically supported treatment movement has led to the identification of several psychological interventions that have received empirical support and has impacted several facets of psychology. One specific area of psychology that has been impacted is clinical supervision. Clinical supervisors will not only need to be familiar with empirically supported interventions, but will need to be able to evaluate a therapist's ability to implement the intervention. One way a supervisor can provide feedback to a supervisee is by using objective measures that help the supervisor evaluate not only if the therapist adhered to the treatment, but also how well he or she implemented the treatment. Unfortunately, few of these measures exist. This study examined the psychometric properties of a measure that is intended to be used by those training and supervising therapists in implementing motivational interviewing, a treatment that has received empirical support for a variety of presenting problems. The inter-rater reliability for this study was excellent as indicated by the generalizability coefficient (ρ2 = .79). The study also provided support that data derived from the MISTS is valid in terms of convergent and discriminant validity relative to the Yale Adherence and Competence Scale. Recommendations for refinement of the measure and future research are discussed.