Over the past 20 years, interest in the impact of space on teaching and learning has grown, and higher education institutions have responded by creating Active Learning Classrooms (ALCs)--spaces designed to promote active, student-centred learning. While ALC research has explored teaching methods, student experience, and student learning, less is known about how teaching in these spaces affects instructors. We contribute to this discussion by investigating teachers' educational development in these spaces. We asked new instructors to reflect on their ALC experiences, exploring their pre-course preparation and their perceptions about themselves, their students, and teaching and learning. Their reflections revealed key differences between knowing and learning: Although all participants knew about and were dedicated to student-centred pedagogy before teaching in the ALCs, teaching in these spaces prompted transformative learning through which they shifted both their behaviours and perceptions about student learning and about their own roles in the classroom.