NFPA 257 prescribes fire and hose stream test procedures that apply to fire window assemblies--including window, glass block, and other light-transmitting assemblies--intended for use in window openings to retard the spread of fire through openings in fire-resistive walls. It establishes controls of the fire test as well as performance criteria and results for reporting procedures.

NFPA 257 is referenced by NFPA 101®: Life Safety Code® when being used to set up a window fire resistance rating in a fire rated wall assembly. The 2007 edition is completely reorganized for clarity and consistency with NFPA's Manual of Style.