Existing buildings sometimes experience ongoing settlement or face the prospect of excessive settlement in the future if a change of building use is required and foundation loadings increase. Several methods of foundation enhancement are available to improve the future performance of existing foundations, One method is to enlarge the foundation dimensions, while another widely used method is underpinning by piles. In this thesis, underpinning methods are reviewed and classified and several case histories around the world are studied to show the application of those methods to existing buildings and historical monuments. It is found that, despite the fact that piles are gaining increasing application in underpinning work, design methods for underpinning piles are not well developed.