Open innovation has rendered increased interest both in practice and research. This paper describes and discusses company-to-company linkages for open innovation. It does so based on a literature review that spans over some different research areas. The paper categorises and discusses such linkages in terms of their openness and how they relate to knowledge management. It concludes that openness needs to be considered in different dimensions that also links to different knowledge management outcomes. The paper's contribution consists of how it connects open innovation research to the general management literature, and how it builds a practical understanding of how linkages between firms can be categorised to aid firms to consider which mechanisms they may choose and why.