The majority of piled foundations will consist not of a single pile, but of a group of piles, which act in the dual role of reinforcing the soil, and also of carrying the applied load down to deeper, stronger soil strata. As will be shown in this chapter, the effectiveness of a pile, in particular in respect of its stiffness, is generally reduced by the proximity of other piles. In recognition of this, current trends are towards the use of fewer, more widely spaced piles, where the reinforcement role of piles is emphasized. Modern analytical techniques allowmore realistic estimates to bemade of the response of pile groups under working loads, thus giving the designer more scope to optimize the foundation layout.