Research has established that tutors often have difficulty with accurately assessing a tutee’s understanding. However, it is a completely open question which characteristics of tutors might affect their assessment. In an empirical study with N = 22 tutor-tutee dyads, we used a methodology developed by Chi, Siler, and Jeong (2004) to examine the influence of the tutors’ content knowledge and beliefs about learning on their assessment accuracy. Results replicated previous research in showing that tutors overestimated a tutee’s correct understanding and underestimated a tutee’s incorrect understanding. In addition, more accurate assessments were positively related with tutees’ learning. Finally, content knowledge had a positive impact on assessment accuracy, whereas beliefs about learning were not strongly associated with assessment accuracy. Thus, assessing a tutee’s understanding seems to be important for the effectiveness of human tutoring. Moreover, the results suggest that the tutors’ assessment accuracy is largely influenced by their content knowledge.