Research indicates that underperforming teachers have a profound impact on students and on principals who struggle to deal with the underperformance. However, the impact on, and responses of, other teachers (i.e. co-workers) is rarely studied, in spite of the importance of teacher collaboration in contemporary education. Therefore, we interviewed co-workers about incidents of teacher underperformance, using the Critical Incident Technique. Our respondents reported various types of underperformance, including student-related and team-related underperformance, as well as task underperformance and counterproductive work behaviours. Dependent on the specific incident, co-workers were more directly or indirectly affected by the underperformance. They expressed frustrations, concerns, and feelings of injustice, not only about the underperformance itself, but also about a lack of response by the school principal. Moreover, we found that co-worker responses depended on how they perceived the necessity, appropriateness, and utility of responding, as well as their responsibility to respond. This was influenced by characteristics of the underperformance, underperformer and co-worker, and leadership and team factors. Implications for educational research, policy, and practice are discussed.