Condition assessment, control of the efficiency and durability of restoration work and service life prediction of reinforced concrete structures need rapid, non-destructive techniques to assess corrosion of the rebars and provide a quantitative measure of the instantaneous corrosion rate. Electrochemical techniques to measure corrosion rates of steel in concrete are based on the determination of the polarisation resistance, R , and in laboratory experiments comparable results are obtained with different techniques. Polarisation resistance measurements on-site have to take into account the following points: non-homogeneous current distribution between counter electrode and rebar network, changes of Rp with daily and seasonal fluctuations in temperature and humidity, influence of the type of instrument and measurement principle. Finally, the conversion of the experimentally measured R p,eff to a specific polarisation resistance Rp* (or the instantaneous corrosion rate) requires a correct compensation of the ohmic resistance and a knowledge of the actively corroding area.