The research presented in this document presents a comparative analysis of the mainstreaming process of Sustainable Urban Drainage Systems (SuDS) in mid-sized and large municipalities in the Netherlands. As a result of the extreme weather conditions presented by the STOWA, deteriorating sewers, and the increase of paved areas in urban environments, alternative solutions (i.e. SuDS) are becoming more relevant. However, the adoption of these innovations is still rather slow. Answers are sought by different scholars, however, for the Netherlands research remains limited. SuDS that are implemented in public space fall under the responsibility of the municipalities. Hence, recent studies have highlighted factors in the implementation process of SuDS for municipalities in general. The biggest municipalities seem to always be leading in research and disparities in how different organizations perceive influencing factors of the implementation process have not been studied yet. The research objective of this thesis is to find to which extent the perceived factors in the main- streaming process between different sized municipalities differ. The methodology consists of a literature review, two case studies, and a focus group. The literature review concerned transition theory which was applied to climate adaptation. A conceptual framework has been developed that describes the mainstreaming process. Different theories were combined in this framework: the Mainstreaming model, four-A framework, Governance Capacity Framework, multi-level perspective. The empirical research applied consists of case studies that have taken place in two Dutch municipalities: Rotterdam and Capelle aan den IJssel. The case studies considered were projects that implemented a new concept for the first time. The main criteria for the selection were the size of the municipalities and their (geo)hydrological boundary conditions. A comparison was made between the results of both studies and cross-checked in a focus group. The data in the case studies were collected through semi-structured interviews and documentation. The semi-structured interviews were based on themes identified in the conceptual framework. The semi-structured interviews were conducted via Microsoft Teams. Examples of questions that were asked were: "What were the main drivers for the project in your city?" and "Do you find that propositions of innovations are well received in your department?". The interviewees were part of these case studies and sampled through a stakeholder diagram and snowball effects. In total 10 participants were interviewed which were part of the case studies. To increase the validation of the case study results, different municipalities were invited to participate in a focus group to evaluate how they identified with the findings. This focus group consisted out The findings of the study imply that the mid-sized municipalities differ from larger municipalities mainly on three factors: • They still have to form climate adaptive strategies • Their allocated budgets are inferior • They lack policies that allow for alternative methods as opposed to traditional sewer management • They lack climate-adaptive experts who are actively involved in projects Furthermore, it was found that knowledge sharing of pilot experiments should be pursued more actively to facilitate the adoption of SuDS. In addition, this knowledge is now mostly catered to engineers, developers, and policymakers. For which it was found that to facilitate adoption, the involvement of asset managers should be pursued too. The research is of conceptual use and is of use for multiple groups. The problem statement has been initiated by VPdelta which is a valorization program that bridges innovative SuDS to the market. It is therefore interesting for them to gain insights into the barriers and driving factors on the side of the clients. For scholars, the research gap of heterogeneity between municipalities has been explored which helps to understand the problem from different perspectives. Lastly, it may be interesting for mid-sized municipalities as recommendations are formed for what they can do to become more climate adaptive.