This chapter presents a technique for assessing dynamic walking stability utilizing a nonlinear time-series analysis and a portable instrument. Main objective is to investigate its usefulness in the assessment of elderly walking. The method consists of a measurement of three-dimensional acceleration of the upper body and an estimation of the Lyapunov exponents, thereby directly quantifying local dynamic stability while walking. The proposed method demonstrates the effects and efficacies of the interventions for the elderly. The experimental results suggested that the method was useful in revealing the degree of improvements on the walking stability. The chapter also investigates the straight level walking of young and elderly subjects.