Autism spectrum disorder (ASD) is a neurodevelopmental disorder and presents as a complex and often puzzling category of conditions. This research evaluates Sensory Integration Therapy (SIT), one of the most popular non-validated treatments for ASD, by comparing the effectiveness to Behavioural Intervention (BI) in altering challenging behaviour. Two studies were carried out to evaluate the effects of SIT and BI on challenging behaviour maintained by environmental variables and automatic reinforcement. Study 1 used an AB counterbalanced group design across 10 participants to compare SIT and BI on behaviours maintained by various functions. Study 2 used a multiple baseline across participants to compare the effects of SIT and Sensory Integration techniques delivered within a behavioural intervention package. The outcome of Study 1found that SIT was not effective at reducing behaviours maintained by environmental variables (e.g., escape from demand, access to tangibles and attention). In contrast, the findings for Study 2 suggest that SIT was somewhat effective in reducing behaviours maintained by automatic reinforcement, and the Sensory Integration techniques were more effective when delivered within the framework of a Behavioural Intervention package.