An increasingly desired outcome of engineering education is the ability to engage in self-regulated learning (SRL). One promising method for the formative assessment of SRL is the reflective diary. There is, however, a paucity of research on the use of reflective diaries in engineering education. To mitigate this gap, we report on a case study where reflective diaries were implemented in a master's course on tissue engineering. The objective of this paper is to explore the potential of reflective diaries for the formative assessment of three central aspects of SRL: conceptions of knowledge, conceptions of learning, and strategies for monitoring and regulating learning. Based on a theoretical thematic analysis of the diary entries, we show that reflective diaries can be used to assess these three aspects of SRL. We discuss ways of providing feedback to students, with a focus on dialogic feedback.