The built environment (BE) affects ecosystems, ecosystem services and human health and well being. While, formally, the BE ranges from the smallest hut to the largest city, this chapter focuses upon the health effects of urban areas, which increasingly are the preferred human habitat. Urban areas have many attractive and beneficial influences to human well-being. But at the same time, many effects of urban areas are harmful to well-being, and many are not even recognized as such. Most publications about these topics have described the effects of the BE separately, on either ecosystems or on human health. The interconnectivity between these two effects relative to BE is rarely studied. This paper focuses on the mutual influence and interactions between three related aspects of the BE which can impact ecosystems and human health: transportation, land use, and life style. It also explores some of the links between the BE, human health, and human security. Transportation, especially when based on systems of private cars burning fossil fuels, is often the most important cause of air pollution in both developed and developing countries. Air pollution has many adverse health effects, including asthma and cardiovascular disease. Transport systems based largely on the use of private cars are a major contributor to global warming and to ecosystem degradation. This occurs directly, as the operation of vehicles releases greenhouse gases causing global climate change which is associated with altered temperature and rainfall patterns and rising sea levels. Warmer ocean temperatures are projected to increase the frequency and intensity of extreme weather events. These effects can also harm cold water fisheries, and otherwise degrade aquatic habitats. The heat-retaining nature of road surfaces and many buildings, together with the loss of vegetation, contributes to "heat islands" sometimes exacerbated by particulate-dense air pollution. Heat islands may create a double burden of pollution as people respond reactively, for example with air conditioning, rather than within urban redesign, such as exchanging black for green surfaces, fewer roads and more rooftop gardens. Transportation is therefore having an impact on ecohealth as well as human health, and these are interconnected. Land use is greatly affected by urban sprawl practices that are responsible for degrading habitats, for altering ecosystem function, and for reducing biodiversity. Sensitive and critical habitats are often fragmented or sacrificed for roads, suburbs and industrial estates. Urban conurbations also pollute reservoirs, ground water, and stream networks with chemicals and pathogens, with numerous adverse health effects. Habitat loss and fragmentation are two of the most direct impacts of development on previously undeveloped land. Habitat fragmentation and an increased proximity of forest, agricultural land and human populations can promote interaction among vectors, pathogens, and hosts, and in some cases lead to increased infectious disease, including Lyme disease. Deforestation continues to increase in many developing countries, in part to supply affluent urban populations. These illustrations demonstrate that not unlike the transportation factor the BE also affects landuse, incurring major impacts on ecosystems and human health. Changes in life style are a direct consequence of the effects of transportation and land use associated with many BEs. High automobile dependency is often characterized by reduced physical activity, and by diminished personal relationships between individuals and groups, a quality known as "social capital". A lack of physical activity combined with excessive caloric consumption commonly leads to overweight or obesity, in turn increasing the risk of many diseases, including Type II diabetes, hypertension, asthma, and cancer. The life style of many BEs exerts a toll on quality of life, including by increased noise disturbance, decreased air clarity and reduced contact with varied and stimulating natural ecosystems. Human security is a widely recognized component of human well-being (Millenium Ecosystem Assessment 2003). Though the World Health Organization (WHO) definition of health does not explicitly include security, the WHO conceptualizion of health is much broader than the absence of physical and mental disease (WHO 1948). It follows that if human health is adversely affected by the BE, then human security will also be reduced, though it is acknowledged that many other factors also influence security. Some of the factors which influence non-health aspects of security, such as the level of crime or interpersonal violence, are also likely to be influenced by the BE, including through the quality and level of social capital and psychological health. As well, the health or wellness of a person is likely to influence that person''s resilience in the face of threat. In general, healthy people will feel more secure. Finally, the community level of health can influence security, by influencing one''s perception of personal disease risk, including in some cases, vector home diseases. In conclusion, adverse effects of the BE, including reduced air and water quality, degraded ecosystems and biodiversity, and the spread and emergence of infectious disease, are relevant to security. The quality of human life and the integrity of ecosystems are affected not only by direct stressors created by the BE which can affect them separately, but also by stressors derived from one or the other, thereby demonstrating the close interconnectedness between the environment and human health. This paper, therefore, highlights the complexity and the interconnections between the BE, ecosystem and human health, and security. In conclusion, there is significant evidence that compact, mixed-use development focused on mass transit can reduce vehicle travel and air pollution from motor vehicles. There is ample evidence that the BE matters to communities- not just for social and economic reasons, but also for environmental reasons of national concern. Issues related to our BE are growing in importance and, if left unaddressed, will make it difficult to meet national and global environmental goals. A richer and more detailed understanding of the BE and sprawl could yield development strategies that address the social, economic, fiscal, political, and environmental impacts of such growth.