The nature of social engagement deficits in children with autism were explored through a series of linked studies. Guiding questions were as follows: i) Are the social deficits present in infancy?, ii) What strategies of adult-child communication facilitate social engagement in preschoolers? and iii) What are the long term effects of musically enhancing these strategies? To address the first question, an interview schedule (the Detection of Autism by Infant Sociability Interview, DAISI) was developed. It was retrospectively employed with parents of children with autism, and parents of learning disabled children without autism as a control group. This revealed social engagement deficits in eye-contact, gestural body-language and interactive babbling, before 24 months. None of these was attributable to learning disability. A subsequent study searched for clinical equivalents of the "scaffolding" role played by parents enhancing their normal infants' social and communicative performance. Videorecorded observations of clinicians and preschoolers with autism during one-to-one play-based assessments were analysed using an observation schedule developed for this study:- Coding of Active Sociability in Preschoolers with Autism (CASPA). This revealed relationships between clinical strategies and episodes of social engagement which confirmed and extended previous research. Facilitative strategies included musical-motoric activities together with self-repetitive communicative turns and/or turns that followed the child's focus of attention. Single case studies were then employed to evaluate clinical interventions (Musical Interaction Therapy) based around the facilitators of social engagement identified by CASPA. Onset of therapy was followed by improvements in social engagement related skills which were sustained for over a year after therapy. The emergence of child acts of teasing and pretend play during and following the period of Musical Interaction Therapy extended beyond the empirical findings of previous interventions. Theories of development in autism were discussed in the light of data from these studies.