An ice detection system, in particular a method, apparatus and controller for detecting icing conditions and the presence of ice formed on a structure. Also, a method, apparatus and controller to detect the prevailing environmental conditions to determine whether or not ice may form on a structure. Two heaters of an ice protection system (experiencing substantially the same environmental conditions) are driven to different temperatures (the first greater than or equal to 0° C., and the second less than 0° C.). A difference in the powers required to drive the two heaters indicates the prevailing environmental conditions. In another embodiment, a heater of an ice protection system is driven and the rate of change of surface temperature is measured over several periods. A substantial deviation from a rate of change of surface temperature, which indicates the presence of ice on the surface of the structure, is detected.