Hello, my name is Dr Steven Papachristou, and I’m an Associate Professor  of Psychology at UCL. Dr Marta Francesconi Hi,  I’m Dr Marta Francesconi, and I am an Associate Professor of  Developmental Psychology at UCL. Dr Keri Wong Hi, I am Dr Keri  Wong, also Associate Professor, Developmental Psychology, and all of us are  from the University College London [pause]. We’re excited to introduce to you to our Special  Issue on "How physical environments promotes the understanding of children and young adults’  psychosocial outcomes." This Special Issue captures the multifaceted and dynamic human  environment relationship across a critical stage of development. It illustrates the  importance of the physical environment in understanding child and adolescent  mental health. Our authors have kindly illustrated the latest work in this field  through original articles, action research, systematic reviews, debates, editorial  perspective and commentaries. This showcases the – and highlights the nuanced  relationship through diverse methodologies, data sources, interdisciplinary  teams and international perspectives. What is clear from the evidence is that the  physical environment impacts psychosocial outcomes early in life. This is true for  both community and clinical populations. So, you may be wondering why this is important.  This is because exposure to adversities, as well as promotive factors early in life  or during critical developmental periods, such as early childhood and adolescence, has  the potential to shape later life outcomes. So, we hope this Special Issue provides helpful  examples of good practice and ways of working together that will inspire you to engage in  future health research that is youth-led and context specific. We also hope that this Special  Issue can encourage us to rethink public health and education policies, urban planning  and design priorities, as well as clinical research and practice, to make sure our young  people are in the centre of all of this work. So, when Psychologists use the term  'environment', they often referring to a very broad range of influences, like  child’s early experiences, family dynamics, social relationships or major life events. And  these are all hugely important for mental health, but when we talk about a 'physical  built environment', we refer to those aspects of our surroundings that are typically  studied by Urban Planners, Architects or Urban Geographer. It includes, but is not limited  to, the physical form of specific dwellings, housing developments and streets and cities.  In other word, it is the man-made physical built environment that we interact with every day.  That could mean the layout of our neighbourhood, the quality of housing, the presence or absence  of parks, but also, how easy is to walk and cycle, and the kind of infrastructure  that shapes how we live and move. We can broadly categorise our physical built  environment into two main types, the urban and rural areas. Urban re – areas sometimes are also  called 'grey spaces', are typically densely built, with lots of buildings, roads and infrastructure.  Rural environments instead are usually more spread out and are often closer to natural  landscape. Within both, urban and rural areas, features that may influence mental health are  green spaces, such as parks, gardens or forest, and blue spaces, which includes natural bodies  of water, like rivers, lakes or the sea. There are also other important environmental  exposures that form part of the built environment, and these are air pollution, noise, especially  from traffic or construction, artificial lights, especially at night. These factor are not always  visible in the same way as buildings or parks, but they are still physical  feature of our environment, and research has increasingly shows that  they can have a real impact on mental health. So, we define what built environment is,  but it is also important to highlight what the built environment does not include. Social  characteristics, such as crime, poverty or the sense of community in a neighbourhood, fall  under the category of social environment. They are extremely important and relevant when we  think about wellbeing, but they are conceptually different from the physical built environment. One  of the biggest challenge in this field is that the environment is made up of many things that are  happening at once. So, for example, someone might live in an area with a very little green space,  lots of noise and high level of air pollution, but it can be hard to figure out which one of these  things is having the strong effect on their mental health, or whether it’s the combination of them,  and of course, people respond to their environment in different ways, so the impact is not the  same for everyone. That is why this is such an important and an interesting area to study.  If we can better understand how our surroundings affect how we feel, we can start creating places  that really support our mental wellbeing [pause]. Dr Steven Papachristou Before we address how the built environment  impacts mental health, it is important to first establish a definition of mental health.  Mental health is no longer seen simply as the presence or absence of a mental illness.  Instead, it’s increasingly understood as a dynamic multidimensional aspect of human  development, one that includes emotional, psychological and social wellbeing. It reflects  how well someone can regulate their emotions, build and maintain relationships, cope with life’s  challenges and function in everyday settings. Importantly, it is now also understood that mental  health exists on a continuum, from positive mental wellbeing to subclinical difficulties to  diagnosable psychiatric condition. This includes what we call 'internalizing symptoms',  like anxiety, depression or withdrawal, but also 'externalising symptoms', such as aggression  or impulsivity. But it also extends to broader experiences, like loneliness, sub – overall  subjective wellbeing and cognitive development. Now, one factor that plays a surprisingly  powerful role but is often overlooked is the built environment. There is a growing body  of evidence that shows that the environments that youth grow up in can act as both risk factors  and protective factors, influencing how children and adolescents develop, adapt and ultimately,  thrive. This happens through various mechanisms, including stress exposure, sensory stimulation,  access to calming or restorative spaces and their chances for social connection. What’s  very important to note is that this Special Issue brings together studies that investigate  these links, but they also reflect the fact that mental health itself is defined and measured  in different ways across disciplines and across studies. Some focus on internalising and  externalising symptoms, while others look at cognitive development, subjective wellbeing  or social connectedness as their outcomes. This diversity of approaches enriches the  field but also highlights the need for a more unified understanding. For example, in one  of the articles included in this Special Issue, Zubizarreta‐Arruti and their colleagues found  that higher air pollution was linked to more internalising problems in children. At the same  time, green spaces around schools were associated with fewer externalising symptoms, and that is  over and above genetic predisposition for mental ill health. And just to give another  example, Brieant et al. showed that in urban settings community cohesion, that is how  connected and supportive a neighbourhood feels, could buffer the negative effects of  socioeconomic disadvantage on youth behaviour. So, what do our Special Issue studies  collectively tell us? They show that the built environment isn't just a background setting.  It actively shapes mental health outcomes, and not just in the short-term, but across  development. If we really want to support children’s mental health effectively, we need  to think beyond individual therapies and even beyond school-based programmes or interventions.  We need to look at environmentally-informed, systems level strategies, once that make sure  that the places that children grow up in are designed to support their emotional,  cognitive and social development. Dr Keri Wong Children and adolescents are our  future. As Researchers who are developing evidence for policymakers to make the best decisions about  our future, we ought to ensure we are providing a robust understanding of the environmental  conditions in which our future generations thrive in. Research evidence on understanding  how the physical built environment impacts children and adolescents’ mental health and  psychosocial outcomes is vital, because early exposure to childhood adverse environments  can impact later life outcomes. Scientists, practitioners and policymakers can then use this  evidence to assess symptoms early, intervene early and hopefully, in turn, prevent poor life  outcomes across different stages of development. In particular, if you think about  your relationship with different environments every day, how you behave, feel  and react, these effects accumulate over time, so it is no wonder that we should also  research more deeply into the spaces that children and adolescents spend time in in  early development. These might be schools, classrooms, community centres, youth  clubs or parks, so we ought to develop the research evidence to show how children  and adolescents interact with their spaces, identify the parts of the built environment  that help promote positive health, so that policymakers can decide how resources  should be allocated to best support this group. Dr Steven Papachristou While there is growing recognition that the  built environment influences mental health, actually translating that knowledge into  meaningful change in design, policy or clinical practice is still quite difficult. One  of the biggest challenges in the field is that it is inherently interdisciplinary. The studies  in this Special Issue sit at the intersection of fields such as architecture, psychology, education  and healthcare, and bringing these perspectives together is not always straightforward. Each  discipline has its own method, the language, jargon and priorities. For example, a Mental  Health Clinician might focus on individual symptoms and diagnosis, while an Architect  is thinking about special flow, lighting, materials, sensory flow. Bridging these  perspectives requires not just shared goals but also a shared vocabulary, and that  is something we still have to develop. Another major issue is data. Even when  we have studies on environmental factors, the data often isn't collected or communicated  in a way that’s accessible across disciplines. We might have air quality metrics, scores for green  space exposure, or even architectural floor plans, but how do we translate those into something  meaningful for a Clinician working with a distressed adolescent, for example? Similarly,  behavioural or symptom data from psychological assessments might not immediately inform design  decisions unless the data are made actionable for Urban Planners or policymakers. This Special  Issue is really helping to move that conversation forward by providing empirical examples where  those silos are starting to be broken down. For example, a study by Robertson and colleagues,  published in our Special Issue, is a great example of what can happen when Architects and  Psychologists co-create with young people. The re-design of youth spaces that is described  in the article was guided by trauma-informed principles and drew on lived experience. That  kind of participatory approach is essential if we want environments that genuinely support  recovery and resilience. Similarly, the work by Brieant et al. illustrates how neighbourhood  level data of socioeconomic conditions and community cohesion can inform our understanding  of risk and protection at the population level. These types of studies help build common ground,  a shared evidence base that different disciplines can engage with, but the challenges are not just  technical, they’re also structural and systemic. Most healthcare and education systems  are still designed around individual level intervention. They tend to  treat mental health as something that is happening only within a person, not  as something that can be influenced by the surroundings. So, even if we have evidence  that a lack of a green space or poor design in youth facilities impacts mental health, the  system isn't always set up to respond to that. This Special Issue also pushes us to broaden  our focus beyond traditional spaces. We often talk about homes, schools, clinics, but what  about community centres, sports facilities, youth hubs? These are places where young  people spend a lot of time in, yet they’re rarely studied from a mental health perspective.  There’s a real opportunity to expand the research lens and consider how those environments can  also be optimised. Ultimately, improving the status quo in mental healthcare for children and  adolescents means recognising that environmental context matters. This Special Issue provides  a foundation for that shift. It doesn’t just highlight the gaps, it also offers models for how  to collaborate across disciplined – disciplines, generate shared data, co-design health  promoting environments with youth, and make those environments part of  the solution, not just the setting. Dr Marta Francesconi So, in conclusion, understanding how the  built environment may influence mental health, particularly in children and young adults, is  a growing and important area of research. One key challenge in this field is that both mental  health and physical environment can be defined and measured in many different ways. These differences  can lead to inconsistencies in findings and make it harder to compare results across studies. This  Special Issue offers a comprehensive overview of current evidence, highlighting how various aspects  of the physical environment, like green and blue space, air quality or urban density, may be  linked to mental health outcomes in young people. By bringing these studies together, we hope  that we can help to identify emerging patterns, clarify key concepts and point to gaps in  the evidence. This deeper understanding can inform the development of intervention and  policies, and aimed at creating a healthier, more supportive environments for children  and adolescents. So, we really hope that you will enjoying this collection of  works that will contribute to shaping future research and guiding practical solutions  that support mental wellbeing from early age.