Urban ecosystems—hosting the highest density of the world’s population—are being recognized to be increasingly vulnerable to climate-related hazards, such as floods and heat waves. Moreover, the vulnerability within urban systems is not only influenced by frequent hazards but also by the changing socioeconomic, political, and governance processes. Given the higher concentration of assets and larger population at risk, climate change vulnerability in urban ecosystems demands prompt attention. This chapter provides a detailed understanding of climate-related hazards and vulnerability in urban ecosystems. First, the basic theories and multiple frameworks of vulnerability are explained. Second, vulnerability to climatic and social hazards is documented to highlight the critical factors for urban vulnerability. At last, the chapter summarizes the proposed risk reduction approaches of nature-based solutions (NbS) and urban sustainability transitions for resilient urban ecosystems.