The transition to Open Science represents a policy challenge, which is best tackled in close cooperation with relevant actors both on local and international scale. This report builds on the exchange of experiences and mutual learning of 13 countries: Armenia, Austria, Belgium, Bulgaria, Croatia, France, Latvia, Lithuania, Moldova, Portugal, Slovenia, Sweden and Switzerland. It provides an overview of various challenges of Open Science implementation across Europe as discussed throughout several meetings in 2017. Its focus is on three topics, all of which are key elements of the European Open Science Agenda: 1) The potential of altmetrics – alternative (i.e. non-traditional) metrics that go beyond citations of articles – to foster Open Science; 2) Incentives and rewards for researchers to engage in Open Science activities; 3) Guidelines for developing and implementing national policies for Open Science. Addressing policymakers and decision-makers in research management, research services and funding organisations this report identifies good practices, lists priorities and outlines potential courses of action for the best possible transition to Open Science.