In urban planning, infill, or in-fill, is the rededication of land in an urban environment, usually open-space, to new construction. Infill also applies, within an urban polity, to construction on any undeveloped land that is not on the urban margin. The slightly broader term "land recycling" is sometimes used instead. Infill has been promoted as an economical use of existing infrastructure and a remedy for urban sprawl. Detractors view increased urban density as overloading urban services, including increased traffic congestion and pollution, and decreasing urban green space. Many conservatives also dislike it for social and historical reasons, partly due to its unproven effects and its similarity with gentrification.

In the urban planning and development industries, infill has been defined as the use of land within a built-up area for further construction, especially as part of a community redevelopment or growth management program or as part of smart growth.

It focuses on the reuse and repositioning of obsolete or underutilized buildings and sites. Such projects can also be considered as a means of sustainable land development close to a city's urban core.

Redevelopment or land recycling are broad terms which include redevelopment of previously developed land. Infill development more specifically describes buildings that are constructed on vacant or underused property or between existing buildings. Terms describing types of redevelopment that do not involve using vacant land should not be confused with infill development. Infill development is commonly misunderstood to be gentrification, which is a different form of redevelopment.

Infill development is sometimes a part of gentrification thus providing a source of confusion which may explain social opposition to infill development.

Gentrification is a term that is challenging to define because it manifests differently by location, and describes a process of gradual change in the identity of a neighborhood. Because gentrification represents a gradual change, scholars have struggled to draw a hard line between ordinary, natural changes in a neighborhood and special, unnatural ones based in larger socio-economic and political structures.

While the exact definition of gentrification varies by scholar, most can agree that gentrification redevelops a lower income neighborhood in a way that attracts higher income residents, or caters to their increasing presence. Peter Moskowitz, the author of How to Kill a City, has more specifically put gentrification into context by describing it as a process permitted by "decades of racist housing policy" and perpetuated through a "political system focused more on the creation and expansion of business opportunities than the well-being of its citizens." Gentrification is most common in urban neighborhoods, although it has also been studied in suburban and rural areas.

A defining feature of gentrification is the effect it has on residents. Specifically, gentrification results in the physical displacement of lower class residents by middle or upper class residents. The mechanism by which this displacement most traditionally occurs is through rental increases and increases in property values. As gentrifiers start moving into a neighborhood, developers make upgrades to the neighborhood that are catered to them. The initial influx of middle class gentry occurs due to the affordability of the neighborhood combined with attractive developments that have already been made in the neighborhood. In order to accommodate these new residents, local governments will change zoning codes and give out subsidies to encourage the development of new living spaces. Rental increases are then justified by the new capital and demand for housing coming into an area. Through increased rents for existing shops and rental units, long time residents and shopkeepers are forced to move, making way for the more new development.