This open access book examines the role of social innovation in cities as an important lever towards climate neutrality. Social innovations are people-centred collaborative solutions, activating the urban ecosystem for change and providing platforms for citizens, civic organizations, the private sector, and governmental institutions to collaborate towards the goal of reducing greenhouse gas (GHG) emissions. The book, based on work developed within the EU-funded project NetZeroCities, which aims to support over 100 European cities to be climate-neutral by 2030, outlines methods, cases, and pathways of social innovation in Europe. It presents social innovation design process pathways through methods, categories of actions, and related cases for developing and scaling social innovations at the urban level, contributing to a systemic approach to tackle the grand challenge of reaching net zero. Within these pages, researchers and policymakers can find examples of cities that have deployed innovative public administration practices by systemically embedding social innovation in varied fields of urban action plans, including energy, citizen engagement, and behavioural change for lowering carbon emissions and increasing well-being.