This thesis deals with product development co-operation between retailers and their suppliers. This phenomenon is studied in the light of the increasing importance of private brands in contemporary retailing. The basic frame of reference for the study is the Interaction and Network Approach to industrial marketing. The study takes knowledge of product development co-operation in the manufacturing industries as its starting point. To this, insights concerning retail companies’ features are added and propositions are developed concerning how product development co-operation "ought" to be carried out in this new context, that is, the retail context. These propositions are then tested in a number of cases of product development co-operation between retailers and their suppliers. The case reports are based on personal interviews with a number of product managers in various retail companies. First the propositions are tested in each case. Then an analysis is carried out across all the cases. Finally, more fundamental considerations concerning the phenomenon are discussed. A proposition finds support in a case where the retailer collaborates with their suppliers in a manner which is consistent with the way manufacturing companies do it; in the other cases that proposition is not supported. Some propositions are supported in all cases, but most of them are supported in some cases but not in others. Some retailers choose for example to co-operate with only a few suppliers, an approach that manufacturing firms are assumed to apply. Both retailers and manufacturers do it this way to keep transaction costs low, and an important reason for retailers is that this increases the opportunities to communicate complex information related to the different requirements which products must meet. Other retail companies choose to co-operate with a larger number of suppliers to encourage competition between them. The study also shows, among other things, that the retailers, like manufacturers, can choose to co-operate in various ways with a supplier depending on how strategically important the product or brand in question is considered to be.