Theoretical equations for the strain coefficient of resistance in metallic thin films are derived. The films considered range from ultrathin, discontinuous films of theoreticalaveragethickness of less than an angstrom to relatively thick films of about 6000 Å which exhibit bulk material properties. The films are classified into five thickness ranges according to the predominating conduction mechanisms. It is found that ultrathin films exhibit strain sensitivity of two orders of magnitude higher than the bulk metal. The sensitivity reduces with increasing average film thickness to a minimum value which occurs in the transition region in which the film conduction properties change from those of a semiconductor to those of a metal. With further increase in thickness the bulk value of sensitivity is approached. The theory is compared with experimental studies and possible misinterpretations in previous studies are discussed.