An electrothermal heater (2) for an ice protection system for an aircraft (1) or the like is provided with improved protection against lightning strikes. The heater (2) comprises a laminated heater mat (3) formed from dielectric layers (51-54) and a heater element (501). A first connector (41) has a first end (415) which is embedded in the heater mat (3) and is electrically connected to the heater element (501) and a second end which extends away from the heater mat for connection to a heater control unit (6). The heater mat (3) further includes first and second conductive ground planes (71, 72) and the first end (415) of the first connector (41) is positioned between the first and second ground planes so as to be shielded thereby and so as to reduce the current induced therein during a lightning strike.

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