Growing numbers of pupils with autism spectrum disorders (ASDs) are attending mainstream schools, and increasing numbers of teaching assistants (TAs) are being deployed into classrooms to support them. Evidence suggests, however, that pupils who have a TA may underperform academically, receive less attention from their teachers and be isolated from their peer group. Issues relating to the deployment and training of these staff, and their relationships with class teachers have been raised as contributing to the negative impact of TAs. However, school factors--such as inclusive school culture and teacher training--have been overlooked. In light of this, the objectives of the current study were to (1) develop understanding of the aspects of school culture that TAs feel contribute to the development of inclusion, and (2) identify aspects of school culture, policies and practices that facilitate or hinder their ability to effectively support pupils with ASD. Fifteen TAs supporting pupils with ASD in four mainstream secondary schools were interviewed. Thematic analysis identified "positive attitudes towards pupils with special educational needs," "school leadership support for inclusion," "collaboration" and "respect" as components of an inclusive school culture. The factors perceived as facilitating or hindering the ability of TAs to effectively include pupils with ASD included "access to expertise," "communication within school" and "teaching staff awareness of ASD." These findings are discussed in relation to the growing literature on inclusive education for pupils with ASD.