The paper describes empirical findings on how openness is realized in practical innovation projects involving different organizations. The purpose of the study is to increase the understanding of how openness should be managed in the various forms of inter-organizational innovation development. The main research interest is in how openness is manifested in developing innovations with different organizations involved in inter-organizational innovation projects. Subsidiary research questions are: ‘What is open?’, ‘To whom is it open?’ and ‘How open is it?’. The study applied qualitative case study methodology, and empirical data were collected by semi-structured interviews with management personnel in 40 organizations in Finland and the Netherlands. The findings reveal that openness in innovation is a multifaceted issue that can have very different meanings in different contexts. In the context of the study, the answer to the first sub-question (‘What is open?’) is obvious. It is the innovation project; its input, process and outcome. As for ‘To whom is it open?’, interviewees made a clear distinction between projects with known actors and projects that may include unknown actors. Answering ‘How open is the project?’, one can distinguish between different projects according to which attribute best describes the openness of the project – readable, usable, or modifiable. Answers to the three sub-questions conceptualize the issue of openness in inter-organizational innovation development and can be considered theoretical conclusions of the study. By combining the answers, five characteristic levels of openness in inter-organizational innovation were derived as practical implications of the study for R&D and innovation management.