Teaching assistants (TAs) in the United Kingdom typically work with students with additional and special needs, including the most challenging and vulnerable pupils, in low paid, precarious roles. However, no research has examined how organisational factors such as job demand, control, and support can influence TAs' wellbeing, despite recent evidence demonstrating the importance of organisational factors to teacher wellbeing. Using a large-scale questionnaire approach, data from 2,957 UK TAs investigated the extent to which job demands, control, role clarity, peer- and management-support, organisational change, relationship quality, and student and parental behaviour contribute to perceived stress. Results show that job demands and control consistently contribute to increased perceived stress in TAs regardless of the phase of school in which they are based. There is also some evidence that TAs experience aggression from pupils and parents. For primary-based TAs, a broader range of factors contribute to stress, including levels of support and negative pupil behaviour. Implications are subsequently discussed, with reference to the TAs themselves and the culture of schools in which they work, as are suggestions of the implications for the children in their care. Suggestions for future research and intervention are also discussed.