UA has been defined as the practice of food production within and around cities [25]. This umbrella concept is used to describe a range of multifunctional and multi-purpose practices that involve different actors and imply a variety of development options [26]. It can include commercial and non-commercial activities [25] and can be operated from intra-urban to peri-urban areas, on public or private land, employing a variety of more advanced (high-tech) to simpler (low-tech) technologies [27], including examples from community gardens to vertical farming [26].

Driven by global consumption and demographic trends, as well as increasing and rapid urbanization, UA is expected to play a crucial role in food production in the coming decades. It has been stressed as enabling shorter and more resilient food supply chains (e.g., with lower transportation costs and a lower food miles impact, as well as lower resource requirements concerning land, water, and fertilizers), supporting several ecosystems services (e.g., cooling, recreation, runoff mitigation) as well as reducing negative environmental impacts [28,29]. Additionally, in times of crisis and economic hardship, including rising energy costs and inflation uncertainty, UA can potentially alleviate poverty and reduce living costs while granting access to healthy food [30,31,32]. Overall, the benefits of UA have been underlined as supporting resilience, as it strengthens the capacity of urban systems to cope with shocks, ranging from climate change to other social, political, and economic challenges [9,29,33].

Alongside these benefits, however, some undesirable effects of UA have also been documented. These primarily relate to potential environmental impacts and risks, namely excessive water consumption [14], potential contamination of aquatic ecosystems, and water quality [15].

The impacts, as well as the challenges of UA, are nevertheless highly contextual and country-specific, dependent on the actors involved, purpose, land use, property, technology, and production system [25,31,34]. For example, in the Global South, UA has been identified as a means of survival and poverty alleviation, while in the Global North, its potential has been related to reducing negative urban development impact on the environment, improving the quality of life of urban communities, as well as improving physical and mental well-being and social inclusion [27,31,35,36]. Nevertheless, the current global challenges have emphasized the fragility of global food supply chains, blurring the South–North distinctions and calling for the re-evaluation of the concept, especially concerning its contribution to close resource loops in cities [31,35,37].

At the same time that the conceptual debate on UA’s definition, purposes, functions, and overall impacts is still ongoing, UA as a sustainability practice is considered more viable when it becomes “circular (using regenerative practices, eliminating pollutants, recycling waste and maximizing exploitation of the inputs used)” [13].

C-E is an “economic system that replaces the ‘end-of-life’ concept with reducing, alternatively reusing, recycling, and recovering materials in production/distribution and consumption processes” [38]. Inspired by natural ecosystems, the concept conveys the possibility of moving away from the “linear” extraction, production, distribution, consumption, and disposal paradigm toward a permanently regenerative economy, focusing on circular flows of reuse, restoration, and renewability, encompassing the entire value chain [39].

While the debate on C-E’s “revolutionary” potential [40,41,42] is still on-going, its possible benefits to achieve SDGs [7,43] have permeated the global agenda. Overall, C-E has received immense attention in the global sustainability discourse: from the European Union Circular Economy Action Plan “(…) establishing an ambitious long-term path leading towards waste prevention and recycling” (EC, 2017a, p. 3), reinforced by the European New Green Deal [44,45], that influenced several Northern European countries C-E strategies and Action Plans [46,47,48]; to Asian legislative frameworks relating C-E to recycling initiatives, eco-parks, and eco-cities [49,50,51]; as well as recognition of the C-E’s potential benefits to the Global South [52,53,54].

As circularity strategies vary substantially depending on the geographies and contexts, the most common definitions encompass activities focused on the 3R principles—reduce; reuse; and recycle [17,39,55]—accepted in academia and C-E practices, and employed in global policy, namely on the European Union, United Nations, and Organisation for Economic Cooperation and Development. This recognition reinforces the potential of the C-E framework when addressing global challenges [49].

As cities are racing toward more sustainable, healthy, and resilient paradigms, C-E has emerged as a possible strategy [56,57], with metropolises such as London and Paris already deploying urban circular roadmaps [58]. At the same time that 80% of the global GDP was generated in cities in 2018, in a linear economy, cities are still “food deserts”, utterly dependent on production from rural areas [59]. The separation between “places of production” and “places of consumption” affects not only food supply but also waste management. When food and organic waste are the endpoints of a linear production-consumption-waste system, cities must contend with an expensive waste management problem [5]. Introducing the C-E concept at a city level can address these challenges, making urban environments more resilient, healthier, resource-efficient, and less dependent on external supply chains (Figure 1).

Cities are potentially ideal test beds for the implementation of C-E strategies. Urban environments concentrate and combine resources, knowledge, and economic activity in a limited geographical area. Cities have the capability to supply the necessary inputs (e.g., waste, byproducts) to develop circularity, while simultaneously implementing strategies that close the loop by recycling such materials and waste [13,59].

There is, therefore, a case to make concerning cities’ role in implementing C-E strategies [58,59,60] and UA part in fostering the transition from linear to circular systems [61].

UA systems can be designed considering regenerative cycles [5]. UA has been found to contribute to resource-efficient food production through the application of circularity strategies such as reduced transport of food products, reduced food waste, reuse of nutrients, use of underutilized spaces, and smart water use [62,63,64]. Transformative processes in UA systems can also enable circular resource flows, reincorporating resources that would otherwise be wasted [12,35,63]. However, despite the apparent interconnections and interdependencies between UA and the implementation of circularity strategies at the city level, a limited overall debate, along with conflicting perspectives, exists on the practical realization of these ideas [5,12,65,66]. The literature points out that, as market forces are the primary driver of land ownership in cities, UA is regarded as a low priority due to preference given to the “highest and best use” that rules land-use planning [67]. UA tends to be viewed by planners as “a placeholder or interim use”. Therefore, embedding agriculture within the fabric of the city on a long-term basis is likely to be limited without specific planning provisions that safeguard the land for its operation, consequently restricting its capacity to adopt circular practices.

Furthermore, the economic potential of UA still lacks robust evidence, jeopardizing its acceptance as a desirable function in cities [68] as well as its impact on the development of circular business models.

Also, the literature has seldom addressed UA’s social externalities, although some studies have pointed out problems such as vandalism [69] and green gentrification [70]. Horst et al. (2017) [68] pinpoint, for instance, that the associated benefits of UA for health, skill-building and jobs, contribution to community development, and food security should be considered with caution as UA may benefit privileged communities, as well as contribute to marginalization and even displacement of socioeconomically disadvantaged households. These shortcomings relate and contrast with the recent imperative for C-E to address the human dimension to achieve significant social objectives, such as enhanced health, improved working conditions, and reduced inequality [71]. Therefore, the conditions that hamper the implementation of CE strategies within UA approaches have been little explored [5,12,66], reinforcing the need to identify obstacles and propose strategies to overcome them to optimize the use of UA to achieve more circularity in cities. A roadmap that uses UA to achieve more circularity in cities needs to identify obstacles of transition.