Our recent work directed at the design, synthesis, characterization and applications of new types of polyborazylene and polyborosilazane polymers is reviewed with a focus on the use of these polymers as processable precursors to BN and SiNCB composites. A design strategy based on the controlled functionalization of preformed polymers with pendant groups of suitable compositions and crosslinking properties has been employed to yield second-generation dipentylamine‐polyborazylene (DPA) and pinacolborane‐hydridopolysilazane (PIN‐HPZ) polymers, which, unlike the parent polyborazylene (PB) and the borazine‐hydridopolysilazane (B‐HPZ) polymers, are stable as melts and can be easily melt-spun into polymer fibers. Subsequent pyrolyses of these polymer fibers then provide excellent routes to BN and SiNCB ceramic fibers.# 1998 John Wiley & Sons, Ltd.