Each year, more than 30 million children and adolescents participate in organized sports in the United States (US), and more than 4.3 million nonfatal sports- and recreation-related injuries are treated in US hospital emergency departments. Two types of activity-related injuries, in particular, that we study in the Bone and Joint Injury Prevention and Rehabilitation Center at the University of Michigan are anterior cruciate ligament (ACL) injuries and head injury and concussion. Knee ACL injuries are occurring in epidemic-like proportions. In the US alone there are approximately 200,000 new ACL injuries at a total cost of over $1 billion per year. ACL-injured patients are at a significant risk of developing early onset osteoarthritis. Exacerbating the concern is the increase in ACL injury rates in youth and adolescents; particularly girls can experience ACL injury rates from two to five times greater than boys in the same sport. In addition, 3.8 million sports- and recreation-related concussions occur in the US each year. Significantly more research is needed, but intriguing findings are surfacing. For example, high school athletes’ recovery times for a concussion are longer than college athletes’ recovery times. Moreover, high school athletes who sustain a concussion are 3 times more likely to sustain a second concussion. Preliminary studies state suggest women have a higher risk for head injury and post concussion syndrome due to potentially weaker neck muscles, different muscle activation patterns, and hormone variations. Three key questions must be answered to understand these and other sports-related injuries: 1) What is the magnitude of the epidemic? 2) What are the causes of injury? and 3) What can we do to prevent injuries? Without a greater awareness of the injury risk and a better understanding of the injury mechanisms, the risk of sport-related injuries may escalate in the coming years.