This chapter outlines the different conceptual frameworks that can be used to better understand the evolving role nature has played in cities. It distinguishes between socioecological systems and urban political ecology, each of which influence how nature has been regarded and treated in different time periods and urban settings. It seeks to provide an overview of these concepts and explain their implications for how urban nature and nature-based solutions are constructed and viewed today as an urban policy issue. The chapter presents different approaches to understanding urban nature, nature-based solutions, and the relationship between nature and cities. It also discusses the emergence of urban nature and nature-based solutions as a response to urban sustainability challenges. The chapter engages with two case studies to illustrate its key messages: Urban Forest Strategy in Melbourne, Australia, and the Eco-Valley of Tianjin Eco-City in Tianjin, China.

Life relies on mutualistic relationships among species, and on the constant rejuvenation of Earth’s materials. Mutualistic cities would do the same thing, enhancing biodiversity, clean air, better soils, fresh water, and stronger communities. Today, however, cities are far from mutualistic. Currently, more than 4 billion people live in cities, and that number is rising quickly. These conglomerations of humanity consume vast Earth resources, and, worst yet, disgorge astonishing amounts of waste into the atmosphere, water, land and sea around them. Unlike "smart cities" that rely on sophisticated technology to monitor and respond to environmental conditions, and unlike "sustainable cities" that stress reduction and reuse, the concept of a "mutualistic city" emphasizes regenerative cycles and virtuous feedback loops. These cities are the key to our future.

Ecologic sustainability assessments are of increasing importance in understanding the physical resource metabolism of urban systems. In Stockholm, the so-called Hammarby Model visualised important synergies in waste and energy flows in the Hammarby Sjöstad urban district and supported improved metabolic thinking. Following the success of this approach, the Eco-Cycle Model 2.0 for the Royal Seaport was developed in cooperation between KTH University and the City of Stockholm. The Eco-Cycle Model 2.0 can take account of more dimensions than the Hammarby Model, including overall and detailed descriptions of resource flows in a lifecycle perspective. Important starting points for the model were (1) global and local challenges concerning the use of resources, with specific relevance for urban development, (2) available models which visualise functions, resource flows, and resource synergies and (3) approaches to material, energy, and water accounting. The primary objective of the model is to show important connections and synergies between resource flows in a modern urban area. Secondary objectives that can be fulfilled in the long term are: to be a tool for the monitoring and follow-up of environmental objectives, to serve as a dynamic tool for the analysis of resource flows, and to contribute to improved urban planning.

This chapter provides an integrative view of the sustainability challenges of urban systems. Thereby, urban metabolism is explored as a promising framework which is often praised as offering a systemic understanding of sustainability challenges in different areas. Nevertheless, this framework still has several shortcomings. To overcome these limitations and to better understand what the urban characteristics are that make cities (un)sustainable and contribute to global (un)sustainability, we take a historical perspective. This is combined with additional pieces of evidence into a general and systemic metabolic view of the effects of urbanisation on social, environmental, and economic sustainability. The chapter concludes by proposing an integrative and transdisciplinary framework to cover the multiple facets of urban sustainability both in terms of spatial scales and from a disciplinary point of view.