The present invention relates to an apparatus and method for detecting the presence of water or ice on a structure, for example on the surface of an aircraft. One or more separate heaters are thermally coupled to a structure (for example on the back of wing leading edge skins). A controller applies power to the heaters to heat the different regions of the surface of the structure. The controller detects the presence of water or ice by comparing the power required to achieve the desired surface temperature, and a reference power, where the reference power is the power required to heat the surface of the structure to the same temperature if the structure were in a dry air environment (i.e. an environment devoid of water). If the power consumed by the heater is greater than the reference power, the presence of water or ice is inferred.