Mat foundations (also known as raft foundations) are a foundation system in which essentially the entire building is placed on a large continuous footing. Mat foundations found some use as early as the 19th century, and have continued to be utilized to effectively resolve special soil or design conditions.

In locations where the soil is weak and the bedrock is extremely deep, floating or compensated mat foundations are sometimes utilized. For this type of foundation, the amount of soil removed and the resulting uplift (on the foundation) caused by groundwater is equalized by the downward forces of the building and foundation. Yet another variation of the mat foundation is to use it in combination with caissons or piles.

A continuous footing supports essentially the whole building. It can act alone or with deeper supports; adding piles or caissons does not eliminate the mat component.