Despite tutors' importance, they often encounter inadequate professional development and support. This study describes the impact of peer observation of teaching activities on tutors' professional development using multiple data-sets over a three-year period. The data was analysed according to three themes: situated learning, reflective practice and conceptual expansion, in order to identify changes to teaching practice. Tutors reported that peer observation of teaching is an effective development activity that encouraged reflection on teaching and, for some, resulted in conceptual expansion and lasting change to teaching. Tutors found the situated aspect of the peer observation activity was an important factor, as it enabled them to observe how other tutors supported their students' engagement with the discipline-specific curriculum. Recommendations for future research and for the professional development of tutors in practice are provided.