Relational view argues that buying companies should integrate supplier resources in new product development (NPD) projects to realise competitive advantages. Due to decoupling of modules and the associated opportunity to allocate development activities to upstream suppliers, modular product architectures are considered to be a driving factor for the integration of supplier resources. Module developments are associated with substantial investments, long-term orientation and risks across product families for the buying firm. Therefore, supplier integration for module development is highly critical. Regardless of the relevance, research has not yet analysed, which supplier competence are considered as antecedents for successful module developments with suppliers. To close the identified gap, this paper analysed case companies from the automotive, agriculture equipment and rail vehicle industry. Technical, organisational and relationship aspects are identified as critical success factors. The results contribute to theory by adding to relational view the finding that competences for successful collaborations differ in accordance to the level of interaction between suppliers and buying firms. Moreover, this study provides a model and an evaluation scheme that helps practitioners to allocate the best suitable development role to suppliers within module developments.