Multidisciplinary practice has become an accepted approach in many education and social and health care fields. In fact, the right to a multidisciplinary assessment is enshrined in the United Nations Convention of the Rights for Persons with Disabilities (United Nations, 2007). In order to avert a "one size fits all" response to particularly heterogeneous diagnoses, such as autism spectrum disorders (ASD), the National Institute for Clinical Excellence (NICE) recommends multidisciplinary input. Yet, multidisciplinarity lacks empirical evidence of effectiveness, is fraught with conceptual difficulties and methodological incompatibilities, and therefore there is a danger of resorting to an ill-defined eclectic "hodgepodge" of interventions. Virtually all evidence-based interventions in autism and intellectual disabilities are behaviourally based. Not surprisingly, therefore, professionals trained in behaviour analysis to international standards are increasingly becoming key personnel in multidisciplinary teams. In fact, professionals from a range of disciplines seek training in behaviour analysis. In this article we brought together a multidisciplinary group of professionals from education, health, and social care, most of whom have a dual qualification in an allied health, social care, or educational profession, as well as in behaviour analysis. Together we look at the initial training in these professions and explore how behaviour analysis can offer a common and coherent conceptual framework for true multidisciplinarity, based on sound scientific knowledge about behaviour, without resort to reifying theories. We illustrate how this unifying approach can enhance evidence-based multidisciplinary practice so that "one size" will fit all.