The open innovation concept has been extensively debated in the last decade and it still ranks very high on the agenda of technology and innovation management scholars. A key issue for innovation managers, especially those working in mature industries where closed innovation projects still represent the largest share of a firm's innovation portfolio (Chesbrough and Crowther, 2006; van de Meer, 2007), is to understand how open and closed innovation can coexist within the same organisation. The paper first develops a framework which identifies the challenges and the barriers a firm working in a mature industry is confronted with when it comes to integrate projects undertaken according to a closed and an open approach to innovation. Afterwards, the results from a multiple case study analysis are presented and discussed. The collected evidence allows us to identify three archetypal modes through which firms, in mature industries, balance open and closed innovation within their project portfolio.