In construction or renovation, underpinning is the process of strengthening the foundation of an existing building or other structure. Underpinning may be necessary for a variety of reasons:

• The original foundation isn't strong or stable enough.
• The usage of the structure has changed.
• The properties of the soil supporting the foundation may have changed (possibly through subsidence) or were mischaracterized during design.
• The construction of nearby structures necessitates the excavation of soil supporting existing foundations.
• To increase the depth or load capacity of existing foundations to support the addition of another storey to the building (above or below grade).
• It is more economical, due to land price or otherwise, to work on the present structure's foundation than to build a new one.
• Earthquake, flood, drought or other natural causes have caused the structure to move, requiring stabilisation of foundation soils and/or footings.

Underpinning may be accomplished by extending the foundation in depth or breadth so it either rests on a more supportive soil stratum or distributes its load across a greater area. Use of micropiles and jet grouting are common methods in underpinning. An alternative to underpinning is the strengthening of the soil by the introduction of a grout, including expanding urethane-based engineered structural resins.

Underpinning may be necessary where P class (problem) soils in certain areas of the site are encountered.

Through semantic change the word underpinning has evolved to encompass all abstract concepts that serve as a foundation.