This chapter examines the following concepts: sandwich structures — concept and design, core material, other z-direction stiffeners, joints, and post-processing operations. Composite structures are composed of a series of sheets or plies that have a particular fiber orientation. Some have unidirectional fibers and some, like cloth, can have two directions of fiber orientation. Still other materials, like mat, can have fibers oriented in many directions but all of them are in a plane because the mat is, itself, planar. All the fibers are held in their orientation by a resin matrix. The term double composite structure describes the concept of a sandwich structure. The double composite’s nature is simply that the face sheets are composites and then the entire sandwich structure is also a composite. Although composites are often molded to near-net shapes (that is, almost finished shapes), trimming, finishing, and assembly into larger structures may be required. The ability to give z direction strength and stiffness to a composite structure greatly expands the capabilities of the material. Composite structures can be made lighter and at lower cost by forming a sandwich construction in which core material is bonded between composite laminate face plates.