Inhibitors, chemical substances that prevent or retard corrosion, are applied to new and existing structures where corrosion has to be prevented. For repair work, inhibitors can be present in paints for the reinforcement, added to repair mortars or applied from the surface. A survey on literature results on inhibitors for steel in concrete is given; in particular calcium nitrite, the migrating inhibitors (MCI or SIKA)™ and MFP (monofluoro phosphate) are addressed. Results from a two year laboratory study on corrosion of steel in presence of migrating corrosion inhibitors (MCI) are presented. The problem of testing the efficiency of inhibitors for steel in concrete in the laboratory and on-site is discussed. A critical evaluation of corrosion inhibitors to be used in reinforced concrete structures with respect to concentration dependence, durability and measurement and control of the inhibitor action is given.