The invention relates to improved structural health monitoring, particularly to structural health monitoring of fibre reinforced polymer composites, more particularly at bonded joints and methods thereof. There is provided a method of structural health monitoring a composite structure (12), wherein said structure (12) comprises at least one non-reinforcing thermal element (18), such as electrically conductive fibres, comprising the steps of applying a stimulus to cause heating of said thermal element (18), removing the stimulus, and measuring the electrical resistance of said element (18) as a function of time, to provide a thermal decay profile (41, 42) indicating whether damage has occured in the composite structure (12).