Nature-based solutions are an increasingly popular means for tackling urban sustainability challenges. The idea that we can use nature to work with us in order to improve our cities is now seen as vital. How the core of this development is the idea that nature is able to provide services and values that can contribute to wider goals for economic, social, and environmental sustainability. This belief that cities should become more sustainable is now very widespread, but it's actually a relatively new idea. It was in the 1980s that the Brundtland Report, Our Common Future, put cities at the heart of the sustainability debate. Since then, a central challenge that cities have grappled with is how they can address climate change, both by reducing the greenhouse gas emissions that contribute to change in the global atmosphere, and by ensuring that they are resilient to the impacts of climate change itself. The Paris Agreement reached in 2015 emphasized the increasing importance of cities in achieving global targets for climate change. The Sustainable Development Goals that were also agreed by the global community in 2015 have shown that addressing urban sustainability also means realizing other important goals like protecting biodiversity, reducing pollution, and enabling equity and social justice. These challenges can sometimes appear to be in conflict with one another and to compete for political attention, public interest, and resources. But as cities have started to take action on climate change and sustainability, it has become clear that some of the most effective responses are those that have multiple benefits. For example, reducing greenhouse gases and improving local air quality, or enhancing resilience and also creating areas of public space in the city, and the importance of finding approaches that can address multiple urban sustainability challenges at the same time has led to a growing interest in nature-based solutions. The term nature-based solutions was coined in the European Union and is an umbrella term for a number of different approaches that use nature to improve urban sustainability, like green infrastructure, green space, restoring rivers, ecosystem services, and ecosystem-based adaptation. For the European Commission, nature-based solutions are defined as solutions that are inspired and supported by nature, which are cost-effective, simultaneously provide environmental, social, and economic benefits, and help build resilience. Such solutions bring more and more diverse nature and natural features and processes into cities, landscapes, and seascapes through locally adapted, resource efficient, and systemic interventions. For the International Union for the Conservation of Nature, a non-governmental organization that promotes nature conservation, nature-based solutions are actions to protect, sustainably managed, and restore natural or modified ecosystems that address societal challenges, effectively and adaptively, simultaneously providing human well-being and biodiversity benefits. While these two definitions vary, both see nature-based solutions as deliberate interventions that seek to use the properties of nature to address societal challenges. In both cases, it is the potential to provide multiple benefits that seems to be key to the value of nature-based solutions. Like for example, managing flooding to securing improved health outcomes for different groups of society or building green roofs on city parks that limit heat stress, city lagoons that store water and permeable surfaces, vegetation and rain gardens to intercept storm water. Yet, despite it attracting increasing interests, the usage of nature-based solutions remains marginal, fragmented, and highly uneven within and between cities, and gray infrastructure and technology-driven solutions continue to dominate urban development. From the design of wastewater systems, to efforts to improve energy efficiency in the built environment. The NATURVATION project funded by the Horizon 2020 Sustainable Cities and Communities Program at the European Union is developing our understanding about how nature-based solutions are currently being used. We have developed the urban nature atlas to show just how nature-based solutions are being implemented in a 100 cities in Europe. Our work shows that it is clear that nature-based solutions are an increasingly popular means of tackling many urban sustainability challenges. But there is more limited evidence that nature-based solutions are becoming mainstreamed within urban planning, policy, and development. Enabling the wider uptake of nature-based solutions means tackling four key issues. The first issue is assessment methods. Although there is a growing body of evidence about the ecosystem services that nature can provide, we have a limited understanding of how this works in an urban context. Many of our assessment tools focus on the ecological benefits of nature, and its economic, social, and cultural values can be neglected. We therefore need new approaches for assessing nature-based solutions that are able to also take these different and sometimes conflicting values into account. The second issue is business models. The benefits of technologies or behavior changes to improve sustainability are relatively easy to calculate, and this has led to business models which can capture these benefits in economic terms and ensure that there is a return on investment for those involved. For example, the rental roof approach has been a popular model for rolling out solar panels in European cities. But nature-based solutions do not come with ready-made business models, and often the value created is distributed between different actors, such as the private firm that instills a Green wall for installation, and the local community that benefits from reduced air pollution. We need to experiment with new business models that can work for nature-based solutions and create the means through which these can be replicated in different urban contexts. The third issue is governance strategies. Municipal governments are important for addressing urban sustainability, yet our work suggests that they cannot act alone. The capacity to address urban sustainability challenges relies on multilevel governance structures, as well as the development of different modes of governance. This means that municipal governance need to work with stakeholders and communities to create the partnerships, resources, plans, and demonstration projects needed to accelerate the uptake of nature-based solutions. We need to examine the different governance strategies being used to advanced nature-based solutions in cities, and consider the ways in which they are able to address conflicts and the inequalities that may emerge from their implementation. The fourth issue is innovation pathways. Mainstreaming nature-based solutions requires that we understand the key challenges and opportunities that are facing projects on the ground. The innovation has to go on a journey from the initial idea and it's demonstration to its wider uptake within policy, industry, and society. Along this journey, getting the assessment of the value of nature-based solutions, the business models required and the governance strategies that can support that uptake will be critical. But perhaps most important is identifying the combination of measures that supports successful nature-based solutions. We call this combination of measures the innovation pathway. By understanding the conditions that enable nature-based solutions to become established in our cities and towns and how that benefits can be shared by society, we hope to contribute to developing sustainable cities for the future.