A continuous process for making a core for a composite panel comprising cutting spaced apart tongues in columns from a metallic sheet, the tongues being foldable about base lines in the columns each base line being separated from the next tongue in the column by a web; folding the sheet to form a castellated structure having continuous side walls constituted by the spaces between adjacent columns of tongues with the webs of adjoining rows respectively lying in spaced apart parallel planes and oppositely folding the tongues of adjoining columns to lie alongside adjacent side walls with their extremities remote from their base lines extending in an appropriate one of the planes. The invention also includes a composite panel comprising the core covered on each side with sheet material as well as a process for making such a panel.