The Report, led by POLITO and co-led by SDU, presents the results of an in-depth investigation into the theoretical foundations of Urban Climate Shelters (UCS), drawing from the literature review and case study review. This Report is a key milestone within MAINCODE Work Package 1 and it aims to expand the knowledge base on climate shelters, defining them as a long-term strategy for climate mitigation and adaptation in urban contexts. Urban Climate Shelters are designed to protect populations, particularly the most vulnerable, during extreme climate events by integrating NbS and co-design processes within a network of safe, inclusive, and accessible public spaces. Finally, the Report positions UCS as promising cooling demonstration projects, focused on transforming schoolyards into climate-resilient spaces to enhance urban liveability, improve human health, and promote regenerative urbanism.