This dissertation focuses on three studies related to sustainable urban wastewater systems and greywater policy. The research aims to address technical, regulatory, and social gaps in sustainable urban wastewater systems and greywater policy through research and innovation, adopting a holistic, systems perspective to realize the water security, environmental, and social benefits of greywater reuse. The main research question is: How can greywater treatment technologies and greywater reuse policies contribute to sustainable urban water systems based on the SETs (Social-Ecological-Technological Systems) framework?The first study conducted a systematic literature review of urban wastewater, covering historical sources, treatment technologies, recycling, and reuse. It summarized the theoretical framework based on the review and developed a conceptual framework for greywater treatment technologies based on the SETs framework, which can support the development of sustainable cities. The second study focused on the public perception of greywater reuse in Phoenix, Arizona, USA. Using a mix of qualitative and quantitative research methods, the study found that city residents have a strong positive perception of and support for greywater reuse, suggesting that the barriers and challenges of public perception can be overcome. The third study examined greywater reuse policies in Arizona and California. It interviewed residents and policymakers and conducted a policy analysis to reveal the implementation benefits, management obstacles, technical restrictions, and challenges of greywater reuse policies in the two states. The study provides recommendations for redesigning greywater policies and improving greywater reuse policies. The dissertation concludes that greywater reuse policies should be informed by the new knowledge from the three studies to establish sustainable water use practices and design greywater reuse regulations and technologies that encourage safe and responsible greywater reuse in urban design. It emphasizes the need to increase economic data on greywater use and public investment to provide better economic costs and benefits, which can help shift interest towards more supportive greywater policy changes. The dissertation highlights that greywater policy is a key factor affecting the sustainability of urban water systems and that greywater treatment technologies and policies can contribute to sustainable urban water systems by addressing the social, ecological, and technological aspects of urban water challenges, supporting the vision of resilient, inclusive, livable, and sustainable water-smart cities.