Students learn more and gain greater understanding from one-to-one tutoring. The preferred explanation has been that the tutors' pedagogical skills are responsible for the learning gains. Pedagogical skills involve skillful execution of tactics, such as giving explanations and feedback, or selecting the appropriate problems or questions to ask the students. Skillful execution of these pedagogical skills requires that they are adaptive and tailored to the individual students' understanding. To be adaptive, the tutors must be able to monitor students' understanding accurately, so that they know how and when to deliver the explanations, feedback, and questions. Before exploring whether in fact tutoring effectiveness can be attributed to tutors' pedagogical skills, we must first ascertain the accuracy with which tutors monitor their students' understanding. This article thus investigated monitoring accuracy from both the tutors' and the students' perspectives. By coding and recoding some data collected in a previous study, the article shows that tutors could only assess students' normative understanding from the perspective of the tutors' knowledge, but tutors were dismal at diagnosing the students' alternative understanding from the perspective of the students' knowledge.