Cartesian three-dimensional braiding as a method of preforming has been investigated. The design of complex and unusual 3-D braids was studied in three parts. These are the grouping of yarns, the fabrication of braids with a complex cross-section, and braids with surrogate material (replacing fibrous tows) added or removed. The grouping of yarns to potentially form hybrid composites was studied via an iterative simulation of the braiding process. Through further analysis of the braid cycles which produce specific yarn grouping, it was found that isolation/insertion rows and columns may be used to quarantine yarns within desired areas of preform cross-section and improve interlacing of the braided structure. In this study, the design of braided composite cross-sectional shape is accomplished through adaptation of the Universal Method. A computer algorithm has been developed which allows the desired cross-sectional shape to be specified and a braid plan for its fabrication automatically determined. A number of 3-D braids, the result of variations or extensions to Cartesian braiding, are also presented. These have been classified as those with axial and transverse yarn insertion, structures with voids, and fillers and fasteners. Braiding equipment has been developed to braid the designed structures. The machines have been used to fabricate four-step braids with transverse, fastener, and filler insertion, special hybrid structures from multiple row and column displacement and multi-step cycles, and uniquely shaped structures with constant and varying cross-sections along their lengths. (C) Kluwer Academic Publishers.