Contact resistance is the hindrance to flow of thermal or electrical energy at the interface where two bodies come in contact by welding, soldering or mechanical movement.

This property is very important in the design, construction, testing and operation of electrical systems and thermal systems, because it causes loss and inefficiency in operation and inaccuracy and unreliability in measurement.

Contact resistance can occur at the interface between different phases of the same material, where two pieces of the same material touch, or where two pieces of dissimilar materials touch. The resistance depends on the actual surface area of contact (constriction resistance), on the presence of oxides or other products of chemical reaction, absorption or adsorption, on thin film tunneling resistance, on the cleanliness and the flatness of the two contact surfaces, and on differences in the conductivity of the materials.

One practical example of the important role of contact resistance in the operation of electrical circuits is in circuit breakers, such as are used for overcurrent protection of equipment and wiring systems and in the ignition system of automobile engines that employ contact breaker points. Breaker points open and close rapidly in synchronization with the rotating engine crankshaft to deliver current with the right intermittent timing to a spark plug. The points can become fouled by oil or dirt, which acts as an insulator between the points. The surface of the points be roughened by metal transfer from one surface to the other caused by current arcing. Arcing may also cause formation of oxide layers on the point surfaces. Changes in the contact resistance of the points over time can cause mistiming of the ignition in the combustion chamber and poor engine performance. This is one reason that breaker-point ignitions have largely been replaced by electronic ignition systems.