Children and young people with intellectual and developmental disabilities (IDD) often present problem behaviours such as aggression and disruption. These behaviours can be successfully treated using Applied Behaviour Analysis (ABA). Unfortunately, despite effective treatment, the relapse of problem behaviour is common, especially when treatment integrity is not maintained. Behavioural Momentum Theory (BMT) suggests that the relapse of problem behaviour is likely to be greater if the behaviour has been reinforced at high rates. Chapter 1 provides an introduction into BMT, treatment relapse, and role-play training and its effect on treatment integrity. Chapter 2 presents a more detailed discussion of BMT and a review of three treatment relapse models (i.e., reinstatement, resurgence and renewal). Chapter 3 reports the results of two reinstatement and resurgence experiments that evaluated the effects of alternating rates of reinforcement on attention-maintained problem behaviour presented by a 16-year-old male with IDD. The two experiments demonstrated that that high rates of reinforcement can lead to greater magnitudes of treatment relapse. Chapter 4 describes a renewal experiment, again using alternating rates of reinforcement, that demonstrated similar findings. Chapter 5 reports the results of a long-term staff training programme that demonstrated that residential staff maintained high levels of treatment integrity following role-play training based on standardised scenarios than staff who received training via traditional methods. Chapter 6 discusses the implications of BMT and treatment relapse for practitioners and provides suggestions for future research.