The increasing differentiation on the supply side of agricultural and horticultural markets has resulted in many governance structure changes between growers and wholesalers. For example, marketing cooperatives are restructured, heterogeneous associations split up in various one-product associations, growers integrate forward into wholesaling, and so on. These developments are analysed with an incomplete contracting model addressing horizontal as well as vertical relationships in a multilateral setting. The interactions between authority, access, and countervailing power in the choice of governance structure are highlighted.