A behavioral assessment system for scoring the behaviors of parents and coaches at youth sports games is described within this paper. The Youth Sports Behavior Assessment System (YSBAS) contains nine behavioral categories describing behaviors commonly seen during youth sports. The developmental process of YSBAS and the observer-training program are detailed. Twenty-eight undergraduates were trained to observe both parents and coaches during youth sports games. These observers attained a mean interrater reliability coefficient of 0.93 (range = 0.79 to 0.98). Results of multiple reliability studies disclosed that high scorer accuracy and inter-rater reliability can be attained using this behavior assessment system. The potential use of this system for evaluating the effectiveness of coaching training programs and parent youth sports orientation programs is discussed. (Contains 1 figure.)