The number of children diagnosed on the autism spectrum (AS) both nationally and internationally has increased dramatically. Development of social-communication abilities is a major difficulty faced by these children. Challenges include attending to others, initiating and responding to social interactions and using and responding to verbal and non-verbal communication. Research has shown the positive impact of supporting parents to adopt facilitative interaction strategies on the social-communication abilities of their children on the AS. However, there is a paucity of research exploring the impact of enhancing teachers and school support staff’s knowledge and use of such strategies. In response to this, a model of professional development was developed by the researcher for this study. The model was informed by the researcher’s social constructivist view of teaching and learning. This study explored the impact of a nine-month professional development initiative on the social-communication behaviours of a teacher, a Special Needs Assistant (SNA) and a young pupil on the AS in five different autism-specific classrooms. The research questions were: 1. Did professional development (PD) in communication-promoting strategies have a discernible impact on how the classroom adults interacted with their young pupils? 2. What were the effects on the social-communication skills of young infrequent communicators on the AS when classroom staff participated in the PD initiative? 3. What were the adults’ perceptions of their participation in the socialcommunication professional development initiative? 4. How did the presence of an external “More Knowledgeable Other” (MKO) impact on the adult participants’ learning within this model of PD?