Falls during stair negotiation are a leading cause of accidental death in the elderly, yet little is known regarding the mechanisms which contribute these falls. Physiologic functional decline associated with aging places older individuals at an increased risk for stair related accidents. The purpose of this dissertation was to begin an investigation into the biomechancal and behavioral factors which may be associated with an increased risk of falling during stair ambulation. Five separate studies were conducted which examined key intrinsic and extrinisic factors thought to be related to balance loss on stairs. All five studies included at least one group of community dwelling adults over the age of 70. while 4 of the 5 studies also involved a group of young adults for comparison. In each of the studies, subjects were tested under various conditions while ascending and/or descending a flight of seven steps. This series of studies demonstrated several important differences in stair descent behavior between young and older adults. The older adults were shown to have more variable minimum foot clearances over the stair treads and edges, which may increase the risk of tripping during stair descent. Although the older adults made lower frictional demands on the stair tread surfaces, thereby slightly reducing the risk of slipping, they were shown to have increased loading rates during stair descent, which may be indicative of decreased stability. Older adults who wear bifocals, and older adults who do not wear glasses used distinctly different patterns of forward head pitch during stair descent, compared to the young adults. The “bifocals”subjects had the greatest maximum forward head pitch during stair decent, which most likely allowed them to direct their gaze through the distance portion of their lenses, but also potentially increased the threat to head and whole body stability. In the study designed to examine the relationship between confidience and stair behavior in middle-old to oldestold adults, stair confidence was shown to be related to the use of safety precautions such as using the handrail. The women in the study had lower levels of confidence in their ability to negotiate stairs safely and exercised greater caution during stair use when compared to the men, even though there were no gender differences in self-reported functional ability or overall mobility self-eflicacy. The results from this dissertation established several distinct differences in the way older adults negotiate stairs which may put them at increased risk of falling. Older adults need to be made aware of the inherent risks associated with stair negotiation, and informed of the approprite ways in which to protect themselves from falling.