This paper advances the concepts of knowledge accession and knowledge acquisition in strategic alliances by identifying supplementary and complementary dimensions to these knowledge transfer modes. Complementary knowledge transfer reflects the similarity of knowledge that the partners have and is conducted in pursuit of higher efficiency and productivity to enhance partner firms' existing competitiveness. Supplementary knowledge transfer occurs when partners each possess distinctive core competences and the information that is acquired or accessed increases the business scope of partners. As knowledge accession entails knowledge amalgamation that does not involve organizational learning, costs associated with the transfer process are lower and trust is easier to establish than in the case of knowledge acquisition. The paper reviews the implications of these transfer modes on trust building in alliances and their costs implications and presents a number of propositions for further exploration. © 2008 British Academy of Management.