In the course of tutoring, tutors have the opportunity to formatively assess a tutee's understanding. The information gathered by engaging in formative assessment can be used by tutors not only to adapt instruction in order to enhance learning but also to form a summative judgment in order to document a tutee's learning after tutoring. We report about an empirical study with 46 tutor-tutee dyads that examined a tutor's formative assessment in response to a tutee's knowledge deficits. The results showed that formative assessment during tutoring supported learning and improved the accuracy with which tutors summatively assessed a tutee's understanding after tutoring. At the same time, formative assessment was more pronounced in response to knowledge deficits that resulted from a tutor's deliberate elicitation of a tutee's understanding than in response to knowledge deficits that tutees spontaneously expressed on their own initiative. In addition, tutors with teaching experience not only caused tutees to express more knowledge deficits but also more often engaged in formative assessment in response to knowledge deficits than did tutors without teaching experience. This difference also explained why tutors with teaching experience were more accurate than tutors without teaching experience in summatively assessing a tutee's understanding after tutoring. Our findings suggest that the learning potential of knowledge deficits that tutees express largely depends on a tutor's formative assessment. In addition, when tutors engage in formative assessment they are able to form a more accurate picture of what a tutee has learned after tutoring.