International research indicates that two to fifteen per cent of teachers perform below standard. These underperforming teachers can have a profound negative impact on their students and schools. As teamwork is indispensable in today's education, co-workers are important stakeholders to include in research on teacher underperformance. With this study, we aim to raise awareness of the experiences of co-worker underperformance, impact on co-workers, and how co-workers perceive their role in responding. The findings of our survey study in primary and secondary schools indicate that the majority of Flemish teachers has recently encountered incidents of teacher underperformance in their teams. This concerns very diverse types of underperformance, including student-related and team-related types. Often, the underperformance is perceived as severe, long-lasting, and having internal causes, and impacting on team members and the team work. Flemish teachers are not convinced they are responsible or authorised to respond to the underperformance or convinced that responding would be useful. Our results also indicate that confrontation and support/advice are the least common co-worker responses. Teachers perceive 'tolerance' as the most common principal response. These findings raise important questions about the role of co-workers in dealing with teacher underperformance and highlight the need to pay attention to these co-workers when studying or addressing teacher underperformance.