An electrothermal heater mat (3) is provided for an ice protection system for an aircraft (1) or the like. The heater mat (3) is a laminated heater mat and comprises dielectric layers (50-58), a heater element (501) and a temperature sensor (507). Each dielectric layer (50-58) comprises thermoplastic material, and the temperature sensor (507) comprises a sprayed metal track (5010, 5012) deposited on a substrate (50, 5019) comprising thermoplastic material. The substrate may be one of the dielectric layers (50) or a separate carrier (5019) which is smaller than the dielectric layers (50-58). The substrate (50, 5019) is laminated to at least a first one of the dielectric layers (53, 58) and the thermoplastic material of the substrate is (i) the same as the thermoplastic material of the first dielectric layer such that the thermoplastic material of the substrate is dispersed or merged into the thermoplastic material of the first dielectric layer or (ii) compatible with the thermoplastic material of the first dielectric layer such that the thermoplastic material of the substrate is fused to the thermoplastic material of the first dielectric layer. Thus, the formation of an undesirable discontinuity at the interface between the substrate (50, 5019) and the first dielectric layer (53, 58) is substantially prevented or minimized.