This work describes the development of magnetic sensing schemes to characterize corrosion of metals in situ. GMR magnetic field sensors are applied to overcome difficulties encountered with traditional corrosion detection methods without damaging the structure. Active and passive magnetic sensing configurations are presented. Passive measurements rely on very near-surface magnetic field measurements. Active magnetic tests rely on the interaction of imposed magnetic fields with corrosion-generated magnetic fields. Although active tests may be carried out at a larger offset distance to the corroding metal, the responses from active tests could not be consistently related to corrosion activity of metals in aqueous environments. Finally the developed magnetic techniques are applied to reinforced concrete specimens. The passive sensing scheme data show promise in identifying areas where corrosion pits initiate; however the corrosion rate is significantly underestimated. The results serve as a basis to propose a robust sensor configuration capable of monitoring corrosion of steel in concrete in the field.