A method of manufacturing a composite sandwich structure, the method comprising a preform assembly method. The method comprises providing a preform (8) comprising at least two sheets (12, 14) connected by a plurality of extensible threads (16), and affixing to the outer face of each sheet (12, 14), a face sheet (18, 22). The adhesion is by a film adhesive (20, 24) placed over the respective sheets (12, 14), and the preform (8) is subjected to pressure or heat to set the film adhesive (20, 24). The method further comprises a method of molding a composite sandwich structure, the method comprising placing a preform (8) in a mold (30), creating pressure differentials between respective chambers (40, 42) within the mold (30), feeding a resinous compound into the mold (30), and allowing the resinous compound to set and cure. The invention also provides composite sandwich structures made in accordance with the methods.