Over the past decades open innovation (OI) practices have gained prominence among both scholars and practitioners as a mean to accelerate time-to-market and reduce development costs of innovations. Thereby scholars have nearly exclusively focused on cross-boundary knowledge flows between the focal firm and (its) external collaborators. This paper argues that such a focus limits our understanding of how multi-business firms with a diverse knowledge base profit from internally applied open innovation practices that in turn provide complementary benefits to OI.We build on the current literature by investigating and describing OI activities that are conducted within the boundaries of the multi-business firm. Based on a multiple case study of 23 collaboration practices conducted across 14 firms, our findings reveal 5 archetypal forms of internally applied open innovation activities. We draw on the literature of cross-divisional innovation and crowdsourcing to highlight how these archetypes differ in terms of their purpose and underlying managerial processes. Multi-business firms regularly rely on internal practices due to lower transaction costs and for stimulating a collaborative culture. We conclude that internally applied OI practices are effectively stimulated by a combination of non-monetary reward systems and purposive integration mechanisms. 97