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. Its detractors view it as overloading urban services, including increased traffic congestion and pollution, and decreasing urban green-space. Many also detract 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. This type of development is essential to renew blighted neighborhoods and to knit them back together with more prosperous communities.

Urban infill Projects can also be considered as a means of sustainable land development close to a city's urban core.

Note that redevelopment or land recycling is a broad term that described development that occurs on previously developed land. Infill development differs in its specificity because it describes buildings that are constructed on vacant or underused property or between existing buildings. Terms describing types of redevelopment that don't 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.