Various methods have been proposed in recent years for the determination of mechanical properties of a material by using instrumented indentation testing. These load and depth sensing indentation techniques imply the measurement of a characteristic load-indentation depth curve by the aid of which numerous materials properties can be extracted. On the other hand in many publications the effect of applied or residual stresses on the results of hardness readings is investigated. Methods are proposed to estimate applied or residual stresses by means of instrumented indentation testing. Based on this obvious inconsistency between these procedures on the use of information of instrumented hardness testing the influence of residual stresses as well as applied stresses on continuous microhardness readings is systematically investigated for steel samples. Experimental investigations were supplemented by finite element simulations of ball indentation tests on equi-biaxially prestressed materials states. These simulations show that the registered force-indentation depth curves as well as the geometry of the indentations are affected by loading and residual stresses in a characteristic way. For hardness values changes of up to 35% are determined with reference to the unstressed initial state.