Productive struggle is crucial to students' mathematics learning. However, it has proven difficult for teachers to sustain struggle when it is productive or make it so when it is not. Studies show that teachers may offer students support when their struggles are productive or even refrain from offering support when students' struggles are unproductive. This suggests that it is challenging to notice struggle, or more specifically, to attend to and interpret evidence of struggle in determining whether and how to respond. Little research, however, has examined teachers' noticing of struggle, leaving teacher educators with limited guidance as they strive to prepare teachers to notice and respond to struggle in ways that will ensure students have sustained opportunities to struggle productively. We examined ten middle-school teachers' noticing of struggle, conceived broadly, while observing students solve challenging problems collaboratively. Teachers attended most to students' struggles to make sense of the mathematics, less often attending to their struggles to communicate their thinking or overcome challenging group dynamics. Teachers were also guided by three narratives about teaching and learning when interpreting the struggles they attended to, which specified that struggle is productive if students are working well together, making progress, or focused on understanding the mathematics. Teachers typically assessed students' struggles as productive, especially when attending to students' struggles to communicate their thinking, yet rarely differentiated their assessments for individual students in a group. We discuss implications for teacher educators striving to prepare teachers to notice and respond to struggle in the classroom.