Agroecology is a promising concept for designing and managing more sustainable agrosystems, but it deeply questions how farmers learn to implement agroecological practices. The building of generic knowledge by scientists is quite insufficient and has to be completed with learning and innovation dynamics on farms. We assume that experimenting agroecological practices on farms is a way for farmers to simultaneously implement and assess new cropping and livestock farming systems, and that the involvement of peers or supporting actors, either scientists or extensionists, could help the experimenting process and favor agroecological transitions. Based on a French participatory research project, we present a cross-case analysis of seven farmers' networks carrying out experiments on crop production and livestock farming. Some experiments were implemented by the farmers themselves on their farms or within a farm network; others were led by technical advisors, veterinarians or scientists. They varied from very formal to much more informal designs and protocols, from individual to more collective experiments. By experimenting, farmers not only assessed the performances of the experimented practices, they also learned how to manage a complex system and observe animals or plants differently, they gained confidence to move to a trajectory of technical change, and in some cases, the experiments stabilized the network for a period by supporting the exchanges between peers. The synergy between individual and collective dynamics was analyzed. Peer-to-peer exchanges provided new ideas and a greater reflexivity on the experiments realized at the individual level. Complementary, the individual experiments were a way to acquire situated knowledge directly usable by each farmer.