The corrosion of steel reinforcing bars in concrete is a significant problem for the US infrastructure, and the need for effective corrosion sensing in structures clearly exists. This paper reviews recent developments in corrosion sensing. First the bases and applications of several different types of embedded corrosion sensors are discussed. We then present a new basis for corrosion sensing based on magnetic field measurement. Magnetic field measurement allows both the extent and rate of corrosion to be measured using active and passive sensing configurations respectively. These two sensing configurations are briefly described. Both active and passive approaches can be applied without excavation of the concrete, so either remote sensing at a surface or internal sensing with an embedded unit are possible. The feasibility of using recently-developed high sensitivity miniature GMR (Giant Magneto-Resistive) sensors in the magnetic sensing configuration is investigated. Initial tests with GMR sensors passive show promise for the passive sensing configuration.