This research aims to explore how school principals determine whether they have low-performing teachers among their staff. Semi-structured interviews were conducted with 24 principals in Chilean public schools. The qualitative research entailed an inductive approach, along with an interview methodology and content analysis to investigate the research questions. Principals rely on three main sources of information to identify low performing teachers: classroom observations carried out by principals and senior leaders, parents' complaints and students' comments. However, there is no single common approach for identifying low-performing teachers, even within the same school district. This study is the first to report on low-performing teachers in Chile from the perspective of school principals.