Thermographic techniques offer distinct advantages over other techniques usually employed to assess damage accumulation and propagation. Among the advantages of these techniques are the fully remote-non contact monitoring and their ability for full field imaging. Due to the transient nature of the heat transfer phenomenon, phase and lock-in techniques are of particular interest in order to increase the resolution of the signal or provide depth discrimination. Last but not least, when a structure is subjected to load, these techniques can be used in order to monitor the irreversible damage phenomena, as manifested by the local heat accumulation in the vicinity of the defect. This eliminates the need for external heat source, as any cyclic loading can induce the heat gradient necessary to pinpoint the defect accumulation and propagation. In the aforementioned context, lock-in thermography has been employed to monitor the delamination propagation in composites and the critical failure of bonded repairs when the materials are subjected to fatigue loading. Lock-in thermography proved successful in identifying debonding initiation and propagation as well in depicting the thermoelastic stress field around purposely induced discontinuities.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.