Modular building systems are buildings that are constructed in sections in a factory before being delivered to a site for assembly. Prefab buildings, modular buildings, modular offices, and manufactured buildings are all terms used to describe modular building products. Unlike conventional buildings, modular buildings are constructed in sections (generally in an assembly line fashion), transported to the site, and put together by a builder on the building site. Modular construction uses many traditional construction materials, but because a modular building construction product is constructed in a factory, the costs and delivery dates are more predictable.

Modular construction provides many advantages over traditional or conventional construction methods. Using a modular building construction product allows builders the advantage of building a facility that is needed today while also allowing for future expansion or relocation. In addition, both site work and building construction work can occur simultaneously, allowing for much shorter construction times with the same level of quality. In addition, with modular building construction products the majority of the construction process takes place at a factory location. This reduces safety and scheduling concerns and keeps disturbances at the final building site to a minimum. Modular building construction products must be built following the same building codes that are required by each state, county and specific locality. In addition, a modular building will be inspected at the assembly plant during each phase of construction. Evidence of this inspection is normally shown by the application of a State or inspection agency label of approval.

Modular buildings systems provide design flexibility, shorter completion times, and reduced costs. With modular buildings or modular offices, buyers still have a wide variety of financing options, ranging from leasing, to purchase, to lease to own projects. According to the Modular Building Institute, although currently less than 5% of construction projects use modular building construction products, given its numerous advantages, modular construction is likely to become a much more popular construction method in the near future.