Experimental aerodynamic information of flying vehicles is obtained through force measurements on captive models in wind tunnels. The most common type of wind tunnel balance today is the strain-gauge balance, which in principle is a spring-balance with electric out-puts. The paper describes the principles of the resistance strain-gauge, and how it can be applied in balances to measure static and dynamic forces. It also discusses proper bridge-wiring to suppress temperature influences and secondary force components. Strain-gauge balances are analog transducers which must be calibrated against known forces and roments.Multi component balances are influened by interac-tions which means that an output of a balance is a function of both a primary and of secondary components. This complicates the process of calibration and a special mathematical model of the balance is used to evaluate calibrations. This model also permits evaluation of aerodynamic forces and moments from wind-tunnel measurements with strain-gauge balances.