Falls represent a severe problem for the geriatric population. Many of the falls attributed to accidents are not solely due to slowness of postural responses, but also to inadequate magnitudes and selections of the appropriate postural responses. The aim of this dissertation was to determine reactive control of posture in the elderly compared to healthy young subjects when responding to either unexpected or predictable perturbations during step initiation. This information could help in developing exercise programs for fall prevention in the elderly. A camera system, a force plate, and eight footswitches were used to record kinematics, kinetics, and timing of footfalls during stepping. In the first study, we studied reactive postural adjustments made in healthy young and healthy elderly adults in response to an unexpected target perturbation. Elderly subjects demonstrated impaired adjustments of lateral propulsive forces and decreased force rates, which worsened as the available time to respond decreased. In the second study, we compared acquisition of novel postural and stepping adjustments in young and elderly adults when responding to a predictable target perturbation and during narrow base of support (BOS) versus wide BOS stepping conditions. Elderly subjects had a slower rate of acquisition of the novel postural and stepping adjustments compared to young adults, suggesting that they require additional practice to achieve the same level of stepping performance as young subjects. Stepping toward a narrow BOS was associated with decreased stepping accuracy and increased time to weight transfer and modify stepping trajectories. In summary, the elderly showed impaired postural adjustments in response to either an unexpected or a predictable perturbation during stepping. Impaired postural control when responding to an unexpected perturbation limits the ability to adjust foot placement during stepping, which is required for dynamic equilibrium and safety. Slower rates of acquisition of novel postural and stepping adjustments may indicate impaired central planning via feedforward control. In both cases, the decreased ability to rapidly plan and execute an effective voluntary modification to a stepping movement may reveal one important cause for the increased risk of falls in the elderly population.