The penetration of a bullet proof vest worn by a first responder in 2003, resulted in the establishment of a research program designed to investigate the long-term stability and durability of all fibers that are used and have the potential to be used in ballistic applications. The Office of Law Enforcement Standards (OLES) of the National Institute of Standards and Technology (NIST) was charged with conducting this program under the auspices of the National Institute of Justice (NIJ). Because of the urgent need to understand the factors that contributed to this unexpected body armor penetration, the initial focus of this research program is on the ballistic fiber material that composed the compromised vest, i.e., poly[(benzo-[1,2-d:5,4-d']-benzoxazole-2,6-diyl)-1,4-phenylene] (PBO) fiber. Since data related to the susceptibility of PBO fibers to environmental attack are limited and sometimes contradictory, this review also focuses on the susceptibility of the benzoxazole ring structure, a major component of the PBO chain structure, to environmental exposure conditions. A somewhat detailed discussion of the formation and degradation chemistry of this ring structure is presented to increase the reader's understanding of how chemical structure and environmental factors may influence ballistic fiber degradation. To balance this discussion, the susceptibility of key linkages in other ballistic fibers to similar conditions is also presented. Therefore, this review seeks to link chemical structure to ballistic performance and provide a fundamental basis for predicting the long-term durability of existing and future ballistic fibers. 9 2006 Springer Science + Business Media, Inc.