Multiple challenges affect the effectiveness of climate change mitigation and adaptation measures, such as accountability, intergenerational justice and developing countries’ increased participation in greenhouse gas emissions. The high complexity of information surrounding models’ assumptions, results, and scenarios related to climate change also presents a challenge, especially when communicating with policymakers. In this context, support tools such as policy platforms can help bridge the science-policy gap by allowing policymakers to understand scenarios and available policy levers, enabling a better understanding of relevant concepts and models or serving as hubs for disseminating best practices and success stories. Available literature often evaluates support tools within the context of use by regular citizens, making it unclear how policymakers perceive support tools and how well they meet their needs, pointing to an important knowledge gap. This thesis explores policymakers’ and policy advisors’ perceptions of the usefulness of climate change mitigation and adaptation (CCMA) policy platforms and the characteristics of such policy platforms they prefer in order to use them as support tools. A collective case study was conducted with ten CCMA policy platforms within the context of the EU-funded Horizon 2020 programme. In addition, interviews (11) and surveys (9) with policymakers and policymakers’ advisors in the Netherlands and seven other countries within and beyond the EU were conducted. Nine characteristics of CCMA policy platforms were identified: Transparency & Credibility of information, Ease of use, Flexibility of use, Accessibility & Portability, Education & Awareness, Communication of complex information, Data visualisation & interactivity, Actively maintained and supported, and Security & privacy. Interviews and surveys show that accessibility, relevance, applicability, and credibility were identified as the primary factors driving the perception of policymakers about the usefulness of CCMA policy platforms and four groups of characteristics were identified with decreasing levels of priority for policymakers: mandatory or must-have (formed by communication of complex information, free and open access to the tool, transparency regarding data sources and limitations, and high level of detail for spatial and temporal data); highly desirable or should-have (formed by availability of training and learning functionalities, availability of detailed documentation on concepts and models, interactive and easy-to-navigate elements, and availability of a web-based platform); ‘nice to have’ or could-have (user stories from policymakers or communities, availability of very recent data, ability to import user data/export results, and ability to modify parameters and run custom analyses), and indifferent (availability of languages beyond English and the ability to use the tool in mobile phones or tablets). Four main recommendations are made to improve the design and use of CCMA policy platforms: Incorporating systematic reviews of existing CCMA policy platforms as part of projects developing such platforms, involving boundary organisations in the development and use of CCMA policy platforms, developing CCMA policy platforms with longer life expectancies and developing CCMA policy platforms flexible for different needs and preferences of policymakers. With these results, new CCMA policy platforms can be developed with a better understanding of how useful policymakers perceive them and what they want from these support tools.