Research has shown that young children with autism can benefit considerably from home-based early intensive behavioural intervention (EIBI) founded on the principles of applied behaviour analysis. Interventions devised by highly trained and experienced supervisory staff are delivered on a daily basis by teams of therapists. Despite the potential for improvement in many areas of children's functioning, variability in outcome is commonplace with only 50% of children at most achieving the high levels of functioning reported for best outcome children. The variability in outcomes observed in home-based EIBI is likely in part to be the result of the quality of therapist performance during intervention sessions. Therapist behaviour in this context is, however, poorly understood. Perceived therapeutic self-efficacy may be one factor responsible for governing therapist behaviour. Exploration of this factor is, at present, hampered by the lack of a valid and reliable measure of perceived therapeutic self-efficacy; the aim of the present thesis was to devise such a measure. To achieve this, it was necessary to identify barriers that therapists considered to impede their ability to deliver home-based EIB!. In the first study, 19 therapists delivering home-based EIBI to young children with autism in the South of England were interviewed. Barriers to intervention delivery included child factors such as challenging behaviour and lack of progress, supervision factors such as being observed during sessions, therapist factors such as emotional reactions to children's behaviour, and factors related to the intervention such as advanced skill targets. Following the identification of barriers to delivering home-based EIBI, two further questionnaire studies sought to clarify the nature of perceived therapeutic self-efficacy and explore predictors of therapists' beliefs. Factor analyses of therapists' responses identified two dimensions of perceived therapeutic selfefficacy relating to (i) teaching a child who is difficult to engage with and (ii) teaching a child whilst being observed. Subsequent regression analysis revealed the former dimension was predicted by therapists' perceptions of their own experiences, general self-efficacy and supervision frequency. There was also evidence to suggest that general self-efficacy beliefs act as a protective factor in this domain when therapists experience difficulties. The latter dimension was predicted by therapists' perceptions of their own experiences, general selfefficacy beliefs and knowledge of behavioural principles. The final chapter presents a summary of these findings, explores future research directions and considers the theoretical and clinical implications for home-based EIBI.