For decades, improved seed varieties, new knowledge and technologies developed by agricultural research institutes to improve agricultural productivity and to enhance food security have been little adopted by end-users owing partially to slow adaptation to needs of small farm holdings, thereby, prompting the implementation of new approaches that account for local actors' needs. Four-year experiments were carried out in the framework of the project "Reinforcement of Partnerships in Agronomic Research in Cameroon" (REPARAC). 12 research projects in partnerships were engaged in action - research in partnerships (A-RP) to promote multi actors' partnerships for innovation. A-RP is one approach to co-validate agricultural research results in which farmers are involved at all phases of innovations' co-production. A-RP arises from the meeting between a research intention carried by scientists and a will of change expressed by field actors. It unfolds in a three phases' cycle with the first phase being crucial. A-RP has a dual objective of "actionable" knowledge production and actors' participation to change and capacity building. The REPARAC project put into actions 95 national and international institutions, involved 147 researchers and university lecturers, 432 farmers' leaders and extension agents. During the first phase, 156 meetings were organized by research teams in two to six months to formalize partnerships by 25 negotiated ethical frameworks. Several other A-RP tools were used to unfold phase II, thus leading to the co-construction of numerous agricultural innovations. The REPARAC case indicates that strong and knowledgeable steering and animation committees constitute the first building block for success that determines the real construction possibilities for propitious functional partnerships. It also depicts that building up multi actors' partnerships requires a lot of time, non negligible competent human and ample financial resources. It is furthermore a risky