The variability in outcomes observed in home-based early intensive behavioral intervention for children with autism is likely in part to be the result of the quality of therapist performance. Therapist behavior in this context, however, is poorly understood. To achieve such an understanding, it will be necessary to specify how factors such as therapist, child and intervention program characteristics, as well as supervision and training provision, influence therapists' interactions with children. This study identified facilitating factors and barriers that therapists considered to influence their capacity to deliver early intensive behavioral intervention to young children with autism. Nineteen therapists associated with various service providers in the South of England were interviewed. In general, responses represented opposite poles of the same construct. For example, child factors such as compliance and competence were considered to facilitate instruction, whereas challenging behavior and lack of progress were perceived to hinder it. These issues are considered in the light of previous research on staff behavior in related contexts. The factors identified suggest specific avenues for questionnaire and experimental research to validate these findings, have implications for routine service provision and may help improve the outcomes of children receiving early intensive behavioral intervention.