Teaching supervision is carried out by principals, and its purpose is to evaluate teachers' teaching practices in the classroom. This study addresses a gap in the teaching supervision literature, which relates to the fact that studies in the teaching supervision literature have overlooked the obstacles principals face when supervising teachers in Saudi Arabia. The study was conducted over 10 days. Using semi-structured interviews, the data was collected from seven female principals in secondary schools. The results showed that the obstacles faced by principals fall into the following two categories: (a) obstacles to supervision caused by some teachers' unhappiness about being observed in the classroom; and because some of them fail to admit to having faults; and (b) obstacles that centre on the classroom environment itself, including noisy students, boring classes, a large number of students, small classrooms, the large number of teachers in a single school, and having to supervise all the subjects. These findings indicate that: (a) principals encounter certain obstacles to supervising teachers which affect the quality of the supervision itself; and (b) supervision planners may be motivated to consider the obstacles faced by principals in their supervision of teachers, so having this information can be crucial for improving principals' performance of supervision in Saudi Arabia.