COOP 2012 is the tenth COOP conference, marking twenty years from the first conference in 1992. In this special anniversary edition we asked researchers and practitioners to reflect on what have been the successes and the failures in designing cooperative systems, and what challenges still need to be addressed. We have come a long way in understanding the intricacies of cooperation and in designing systems that support work practices and collective activities. These advances would not have been possible without the concerted effort of contributions from a plethora of domains including CSCW, HCI, Information Systems, Knowledge Engineering, Multi-agent systems, organizational and management sciences, sociology, psychology, anthropology, ergonomics, linguistics, etc. The COOP community is going from strength to strength in developing new technologies, advancing and proposing new methodological approaches, and forging theories.