The reliance on teaching assistants (TAs) for inclusion is discussed in relation to the quality of instructional support. Swiss policy stipulates two distinct models for TAs working in mainstream classes: they can either be employed to work with a particular pupil with special educational needs (SEN) or be employed as a general aide to provide support for the whole class. This article describes a study investigating teachers' and TAs' interactions with pupils during individual seatwork based on observational video data and examining how these reflect the roles foreseen for TAs. The analysis is based on videos of 90 minutes in each of the 31 primary school classes with TA and teacher being present. Results of the video analysis of individual seatwork demonstrate that TAs interact longer with pupils with SEN and pupils with lower attainment levels than teachers do. The analysis of the content of instructional support reveals that TAs often use transmissive approaches, i.e. telling a pupil how to tackle a task or even provide the correct answer, whereas co-constructive support, such as scaffolding, is less prevalent. The findings are discussed regarding their relevance for policy and the development of practice.