The design and construction of foundations, which are those parts of structures that transfer loads to the underlying ground, is an ancient art; yet, only within the last 45 years has it passed from an empirical stage to a scientific stage which is marked by increasing understanding of the physical laws involved. The development of the relationships between predicted and actual performance has been important, for these relationships are influenced by the characteristics and behavior of the subsoil through application of the principles of soil mechanics. The common types of foundations are footings, rafts, piers, piles, and caissons. The choice of foundation types and structural material to be used depends upon the type of structure, the loads it must carry, the subsurface conditions, and the cost relative to the superstructure. Design of the foundation should be evaluated with respect to such factors as bearing capacity and settlement, possible seasonal volume changes of the subsoil, scour if the soil is subject to running water, and uplift or lateral forces. Improvement of the subsoil may sometimes be achieved by appropriate treatment, such as increasing the density by driving piles or by vibroflotation, promoting rapid consolidation through sand drains, or stabilizing by chemical or thermal means.