A multiple case study has been carried out of four teacher groups who engaged in collective lesson design, observation, and reflection to support their professional learning. The teacher groups were examined on what and how they learned from their collaboration over time. For each meeting, teachers' learning logs and transcripts were analysed. The results show that the groups differed in the amount, consistency, and stability of self-reported learning outcomes throughout the meetings. Differences between groups also relate to the number and type of dialogic moves between the teachers within each group. A main conclusion includes that challenging each other in their dialogues supported teachers' learning. Additional explanations of learning outcomes relate to teachers' collective participation and facilitation in the group.Keywords: teacher professional learning; teacher collaboration; collaborative learning activities; teacher dialogue; secondary education