Large structural composite sandwich panels (approx. 2x4 m2) were found to have cracks in the honeycomb core. The core was made out of a number of Nomex honeycomb blocks having different densities. These were first machined and adhesively bonded into the final shape. The CFRP skins were then added and the sandwich panel was cured in an autoclave applying elevated temperature and pressure. Acoustic emission monitoring was done during 17 hours of processing in the autoclave using a six channel AE system. AE events from cracking of the honeycomb core were registered and could be identified in time and space. The results from AE monitoring were very useful for modifying the process parameters and overcome the cracking of the core.