In Sweden, it is expected that more than 90% of the population will live in cities by 2040. Therefore, cities have an important role in sustainable development to ensure high quality of life in the face of climate change and social challenges. This thesis uses the framework of Urban Living Labs as a form of experimentation to analyse how evaluation and learning in municipalities take place. Districts identified as sites of experimentation in the six mid-sized municipalities Umeå, Gävle, Jönköping, Borås, Örebro, and Lund are studied using a multiple case study approach. Secondary data is discussed together with findings from interviews. The results show that the aims and challenges of experimentation are similar in all cases. Municipalities aim at increasing their attractivity with these extraordinary districts, but they also contribute to sustainable development through innovation in different sectors (e.g., energy, construction, social life). The importance of structured evaluation for learning is recognized, but only few have (as of now) developed a framework for this. Double-loop learning (or reflectivity) is hinted at in the interviews, but for most projects, it is too early to identify whether it will take place. Experiments are contributing to sustainable development on a larger scale mostly through interactions in networks. Personal Networks, as well as international networks (e.g., initiated by the EU), have a special role as these provide incentives for experimentation, quick informal exchange, as well as spread findings and knowledge to municipalities outside of Sweden.