Background: The assessment of intervention integrity is essential in psychotherapeutic intervention outcome research and psychotherapist training. There has been little attention given to it in mindfulness-based interventions research, training programs, and practice. Aims: To address this, the Mindfulness-Based Interventions: Teaching Assessment Criteria (MBI:TAC) was developed. This article describes the MBI:TAC and its development and presents initial data on reliability and validity. Method: Sixteen assessors from three centers evaluated teaching integrity of 43 teachers using the MBI:TAC. Results: Internal consistency (a = 0.94) and interrater reliability (overall intraclass correlation coefficient = 0.81; range = 0.60-0.81) were high. Face and content validity were established through the MBI:TAC development process. Data on construct validity were acceptable. Conclusions: Initial data indicate that the MBI:TAC is a reliable and valid tool. It can be used in Mindfulness-Based Stress Reduction/Mindfulness-Based Cognitive Therapy outcome evaluation research, training and pragmatic practice settings, and in research to assess the impact of teaching integrity on participant outcome.