Abstract: A reinforcing core structure for a composite panel includes a body having a plurality of generally parallel, alternating ridges and grooves, in which walls extending between the ridges and grooves have a corrugated surface. The resulting core structure has an open honeycomb geometry that may be employed in various applications to provide a composite panel exhibiting an exceptional strength to weight ratio. In accordance with certain preferred embodiments, the panels may comprise reinforcing core structures fabricated from fibrous bodies containing a binder material to provide inexpensive structural members exhibiting excellent mechanical properties.