A multiple baseline research design across sample and settings was utilised to examine theeffects ofcanter's Assertive Discipline Program (1979) on teachers' ratesof verbal approval and disapproval, student on task behaviour, and the frequency of students' disruptive behaviour. Classes from Years three, four and five in five separate British primary schools were the participants (in total 15 teachers and 120 students) of the study. During daily mathematics lessons the teachers and a target group of eight students from each class were video-recorded for 30 minutes. These observations were undertaken prior to and following the teachers completing a workshop in Assertive Discipline. Data were analysed in two ways. First, a one-way repeated measures analysis of variance was calculated to test for significant differences in pre-and postgroup means. Second, data for individual teachers and classes were analysed using DMITSA 2.0 (Crosbie & Sharpley, 199 1 )which is astatistical programme specifically developed to analyse data from interrupted time-series designs. Overall, teacher approval increased and teacher disapproval decreased, student on-task behaviour increased, and the frequency of disruptive behaviour decreased, all significantly. Analyses of individual teacher trends showed, however, that not all teachers' and students' behaviour changed significantly.Assertive Discipline (Canter, 1979) is a widely utilised classroom behaviour management programme which like many others draws upon behavioural psychology as its basis. Briefly, Assertive Discipline comprises one, six-hour in-service course, utilising videotapes, text materials and activities which teach participants to develop a plan for classroom discipline. This plan includes a small number of observable rules, use of clear statements to students regarding their behaviour, a hierarchy of consequences for disruptive behaviour, a variety of reinforcers for appropriate behaviour. a visual display of the formulated plan, informing students and parents of the plan, and obtaining administrator support for the plan (Canter & Canter, 1976).