Background: Diminished control of standing balance, traditionally indicated by greater postural sway magnitude and speed, is associated with falls in older adults. Tai Chi (TC) is a multisystem intervention that reduces fall risk, yet its impact on sway measures vary considerably. We hypothesized that TC improves the integrated function of multiple control systems influencing balance, quantifiable by the multi-scale "complexity" of postural sway fluctuations. Objectives: To evaluate both traditional and complexity-based measures of sway to characterize the short- and potential long-term effects of TC training on postural control and the relationships between sway measures and physical function in healthy older adults. Methods: A cross-sectional comparison of standing postural sway in healthy TCnal ve and TC-expert (24.5±12 yrs experience) adults. TC-nalve participants then completed a 6-month, two-arm, wait-list randomized clinical trial of TC training. Postural sway was assessed before and after the training during standing on a force-plate with eyes-open (EO) and eyes-closed (EC). Anterior-posterior (AP) and medio-lateral (ML) sway speed, magnitude, and complexity (quantified by multiscale entropy) were calculated. Single-legged standing time and Timed-Up- and-Go tests characterized physical function. Results: At baseline, compared to TC-nalve adults (n=60, age 64.5±7.5 yrs), TCexperts (n=27, age 62.8±7.5 yrs) exhibited greater complexity of sway in the AP EC (P50.023), ML EO (P,0.001), and ML EC (P,0.001) conditions. Traditional measures of sway speed and magnitude were not significantly lower among TCexperts. Intention-to-treat analyses indicated no significant effects of short-term TC training; however, increases in AP EC and ML EC complexity amongst those randomized to TC were positively correlated with practice hours (P50.044, P50.018). Long- and short-term TC training were positively associated with physical function. Conclusion: Multiscale entropy offers a complementary approach to traditional COP measures for characterizing sway during quiet standing, and may be more sensitive to the effects of TC in healthy adults.