The early 21st century has seen a considerable increase in both the number and presence of teaching assistants (TAs) and learning support staff in classrooms. In the United Kingdom and elsewhere, TAs have assumed responsibility for teaching lower-attaining pupils and especially those with special educational needs or disabilities (SEND). This drift has occurred in a largely uncritical way and has attracted little attention because of the attendant benefits additional adult support has for teachers. However, evidence from research in the United Kingdom and the United States have revealed troubling and unintended consequences of this arrangement in terms of impeding pupil progress and increasing the likelihood of pupils’ dependency on adult support. Of particular concern are research findings that show how a high amount of support from TAs for pupils with high-level SEND leads to a qualitatively different experience of schooling compared to pupils without SEND, particularly in terms of having fewer interactions with teachers and peers. Heavy reliance on the employment and deployment of TAs to facilitate the inclusion of pupils with often complex learning difficulties in mainstream settings can be seen as a proxy for long-standing and unresolved questions about how teachers are prepared and trained to meet the learning needs of those with SEND and the priority school leaders give to SEND. Future efforts to meaningfully educate pupils with SEND in mainstream schools must attend to teachers’ confidence and competence in respect of this aim. In addition, extensive and collaborative work with schools in the United Kingdom is offering a more hopeful model of how TAs can supplement this endeavor. Improving how teachers deploy TAs and how TAs interact with pupils, together with addressing persistent problems relating to the way TAs are trained and prepared for their roles in classrooms, schools can unlock the potential of the TA workforce as part of a wider, more inclusive approach for disadvantaged pupils.