The defensible space theory of architect and city planner Oscar Newman encompasses ideas about crime prevention and neighborhood safety. Newman argues that architectural and environmental design play a crucial part in increasing or reducing criminality. The theory was developed in the early 1970s, and he wrote his first book on the topic, Defensible Space, in 1972. The book contains a study from New York that pointed out that higher crime rates existed in high-rise housing projects than in low-rise complexes. This, he concluded, was because residents felt no control or personal responsibility for an area occupied by so many people. Throughout his study, Newman focused on explaining his ideas on social control, crime prevention, and public health in relation to community design.

As defined in Newman's book Design Guidelines for Creating Defensible Space, defensible space is "a residential environment whose physical characteristics—building layout and site plan—function to allow inhabitants themselves to become key agents in ensuring their security." He goes on to explain that a housing development is only defensible if residents intend to adopt this role, which is defined by good design: "Defensible space therefore is a sociophysical phenomenon," says Newman. Both society and physical elements are parts of a successful defensible space.

The theory argues that an area is safer when people feel a sense of ownership and responsibility for that piece of a community. Newman asserts that "the criminal is isolated because his turf is removed" when each space in an area is owned and cared for by a responsible party. If an intruder can sense a watchful community, he feels less secure committing his crime. The idea is that crime and delinquency can be controlled and mitigated through environmental design.

There are five factors that make a defensible space:

The concept of defensible space is controversial. A United States Department of Justice experiment in Hartford, Connecticut, closed streets and assigned police teams to certain neighborhoods. New public housing projects were designed around ideas of limited access to the city, but Hartford did not show any dramatic drop in crime.

The intention of physical features is to create a sense of territorialism in community members which will ensure a safe living environment for those that care for it. Defensible space works with a hierarchy of living and community spaces. According to the theory, housing developments that evoke territorialism are "the strongest deterrents to criminal and vandal activity." Housing should be grouped in such a way that members feel a mutual benefit. Also to deter crime, areas should be defined for function, paths should be defined for movement, outdoor areas should be juxtaposed with homes, and indoor spaces should visually provide for close watch of outside areas.

Newman holds that through good design, people should not only feel comfortable questioning what is happening in their surroundings, but they should feel obligated to do so. Any intruder should be able to sense the existence of a watchful community and avoid the situation altogether. Criminals fear the likelihood that a resident, upon viewing the intrusion, would then question their actions. This is highly effective in neighborhoods that cannot afford a professional crime watch.

The defensible space theory is applicable to any type of planned space. From low density housing to high rises, the key is the development of a communal area in which residents can "extend the realm of their homes and the zone of felt responsibility." Circulation paths and common entry are important aspects of defensible design as well. Residents must also feel a need to extend their protective attitudes to locations where property and urban streets and surroundings connect. The interfacing between private property and community space should be protected similarly.