There are an estimated 1.6 – 3.8 million sports- and recreation-related concussions in the US each year, and adolescents are particularly vulnerable due to high participation in sports and prolonged recovery periods when injured. Female athletes have higher concussion rates in sex-equivalent sports such as soccer; however, female athletes are understudied with only 20% of concussion and head kinematic sensor studies including female athletes. There is growing concern for the neurological outcomes associated with repetitive head loading in contact sports. In this dissertation, the effects of repeated head loading were investigated in adolescent male and female athletes through four research aims. Sensor error (i.e., intrinsic error, coupling error, and false positive and negative recording) was quantified for two head kinematic sensors, and best practices were established for collecting and interpreting on-field head kinematics in subsequent studies (Aim 1, Chapter 2). Head acceleration event rate, mechanism, and kinematics were quantified in adolescent male and female soccer, basketball, lacrosse, and field hockey athletes (Aim 2, Chapter 3). The effects of repeated soccer heading were assessed using a comprehensive suite of clinical and objective measures of neurophysiological function (Aim 3, Chapter 4). Global kinematics from two soccer heading directions were applied to a finite element brain model to estimate tissue-level measures of injury such as strain and stress and regional sensitivity to head loading (Aim 4, Chapter 5). Soccer had the highest head acceleration event rate, and males consistently had higher rates than females. In lacrosse, female athletes experienced higher peak linear and angular accelerations, contributing to the debate over the use of helmets in female lacrosse to reduce head kinematic magnitudes. Oblique soccer heading (i.e., redirecting the ball to the side) resulted in higher angular accelerations and higher stresses and strains throughout the brain compared to frontal headers. Overall, completing 10 soccer headers did not cause deficits in vestibular, ocular, or autonomic function, which can help inform soccer heading limitations in youth soccer players. This unique combination of clinical and biomechanical approaches in laboratory and real-world studies provided critical information to inform sex- and sport-specific rule and equipment changes to reduce head loading.