The inter-rater reliability of university students' evaluations of teaching quality was examined with cross-classified multilevel models. Students (N = 480) evaluated lectures and seminars over three years with a standardised evaluation questionnaire, yielding 4224 data points. The total variance of these student evaluations was separated into the variance components of courses, teachers, students and the student/teacher interaction. The substantial variance components of teachers and courses suggest reliability. However, a similar proportion of variance was due to students, and the interaction of students and teachers was the strongest source of variance. Students' individual perceptions of teaching and the fit of these perceptions with the particular teacher greatly influence their evaluations. This casts some doubt on the validity of student evaluations as indicators of teaching quality and suggests that aggregated evaluation scores should be used with caution.