The electrical resistance of carbon fiber bundles alone and embedded in epoxy was measured under tensile loading. Electrical resistance of bundles increases relatively slowly in a stepwise manner with increasing strain. In carbon fibers impregnated with epoxy the change of resistance is affected by the thickness of epoxy layer.and surface treatment of the fibers. Volume fraction and initial number of fibers in epoxy determines the piezoresistance properties of the specimen. Strain monitoring by piezoresistance changes in bundles and composite specimens yields immediate information on damage inside these structures.