Marketing co-operatives can strengthen countervailing power of producers and help to co-ordinate (agricultural) supply chains. According to many authors, one of the means of increasing the competitiveness of co-ops can be traced back to trust and informal connections existing between the members and the cooperative (management), as well as among the members. Using New Institutional Economics’ theories and the “co-operative identity” concept as theoretical background, this review paper analyses the importance of trust, as well as the role of the co-operative principles as formal-legal securities of trust (development) in agricultural marketing co-operatives.