The electrical and mechanical properties of carbon nanotube 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 carbon nanotube (CNT) doped epoxy exhibited a 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 fibre cross-ply laminates with nanotube doped matrix and resistance changes. It was shown that resistance changes 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 resistance changes 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