Assessing students' understanding is central for teachers. While research has focused on factors affecting accuracy as a main performance measure of diagnosing, less is known about teachers' diagnostic process. This study investigated the diagnostic process of pre-service teachers in a simulation using a person-centered approach. We examined the frequency of the diagnostic processes describing, explaining, and decision-making as well as their relation to dispositions and diagnostic performance. Findings show that participants’ varying engagement in the diagnostic process is related to different levels of knowledge, task value, and accuracy. We discuss consequences for the adaptive support of learning to diagnose.