Abstract: A lightweight, flame-retardant, multilayered composite structure having at least the following components: a thermoplastic foam core having two opposing surfaces; a thermoplastic adhesive film on at least one of the opposing surfaces of the foam core, one or more composite layer(s) on each adhesive film. The composite layer(s) is/are composed of reinforcement fibers embedded in a thermoplastic polymer or thermoset resin matrix. Adhesive bonding is effectuated by the interleaving thermoplastic adhesive film interposed between the thermoplastic foam core and the adjacent composite layer. The thermoplastic adhesive film is formed of a thermoplastic polymer composition having a Tg of at least 20° C. lower than the Tg of the foam core material.