Carbon fibers are widely used as a reinforcement in structural composites. The use of these fibers that are already in the composite for strain sensing eliminates the need for strain gages, optical fibers or other sensors, thus decreasing cost and improving durability. For this purpose, the electromechanical behavior (change in DC electrical resistivity upon strain) of the fibers was investigated. A single bare carbon fiber increases its electrical resistance (not resistivity) reversibly upon tension due to changes in dimensions, such that the gage factor (fractional change in resistance per unit strain) in +2. A single carbon fiber embedded in epoxy (a common matrix) has its volume electrical resistivity increased by 10% after curing at 180°C and subsequent cooling of the epoxy, due to the compressive residual stress resulting from the thermal contraction mismatch between fiber and epoxy. Subsequent tension of the embedded fiber at its two exposed ends decreases the residual stress and causes the fiber resistivity to decrease back to its value before embedding, such that the resistivity decrease is reversible, with gage factor -17. Excessive tension causes the resistivity of the fiber to increase, due to damage. Thus, carbon fiber in epoxy is a sensitive piezoresistive strain sensor.