The electrical and mechanical properties of carbon nanotube (CNT) doped epoxy resin and composites based on this matrix were studied. The investigation was carried out on neat nanocomposites and on structural composites i.e., when the nanocomposite is used as matrix in composite materials reinforced with long continuous fibers. Tensile tests showed that CNT doped epoxy exhibited clear piezoresistive behavior. It was, however, also shown that geometrical changes of the specimen also contribute significantly to resistance changes during tensile loading. Particular effort was made to establish the relations between transverse cracking in glass fiber cross-ply laminates with nanotube doped matrix and changes of electrical resistance. It was shown that changes of electrical resistance during tensile loading of composites containing CNT doped matrix gives highly relevant information about the damage state of the material. In an unloaded state the resistance change is proportional to the relative change of stiffness. This work demonstrates that there are three different mechanisms, which contribute to changes of electrical resistance of a composite specimen subjected to tensile strain. These three mechanisms are: (a) geometrical changes of the specimen (b) piezoresistive material response, and (c) accumulation of micro-damage. © 2009 SAGE Publications.