Agresta, CE, Church, C, Henley, J, Duer, T, and O'Brien, K. Single-leg squat performance in active adolescents aged 8–17 years. J Strength Cond Res 31(5): 1187–1191, 2017—More than 30 million U.S. adolescents participate in sport and exercise. Lower extremity injury from sport participation accounts for up to 89% of injuries each year. The single-leg squat (SLS) is a simple clinical tool that assesses lower extremity mechanics often associated with injury risk. To date, there is limited information regarding SLS performance in healthy children. Such information could be useful when assessing youth athletes to determine if mechanics demonstrated are different than is to be expected and puts them at the risk for injury. Furthermore, maturity status is thought to influence motor performance. Currently, there is no information regarding the influence of maturity status on SLS performance in adolescents. The purpose of our study was to determine SLS performance in relation to age and maturity level in adolescents. Forty-five children aged 8–17 years were videotaped performing a series of 10 squats using a standardized protocol. Standing height, seated height, and leg length measures were collected. Investigators scored the SLS test using specific scoring criteria. Adolescents were categorized into 3 maturity levels using a peak height velocity calculation. Multiple linear regression analyses and analysis of variance were used to analyze the data. Chronological age was a significant predictor of SLS performance with younger children having poorer SLS scores. Coaches and trainers should consider the chronological age of the youth athlete when assessing SLS performance. Furthermore, tailored training programs by age may help to address faulty areas, like single-leg stability, and improve overall functional performance.