We assessed the reporting of treatment integrity in school-wide prevention programs in K-12 schools. This review was designed to determine (a) the extent to which treatment integrity was reported in school-wide prevention and intervention programs and how the reporting varied by research design, year, and journal; and (b) the procedures (e.g., method, frequency, informant) used to collect treatment integrity data. Results indicated that fewer than half of the studies in the review (n = 36, 45.6 %) measured and reported treatment integrity. Those studies reporting treatment integrity often used multiple methods and informants. Reporting treatment integrity in this body of literature has increased steadily over time.