The incidence of anterior cruciate ligament (ACL) injuries rises over time. Owing to earlier participation in high-intensity sports in children, longer life expectancy, increased physical activities in the elderly, and improvement in diagnostic resources, the injury effects a wider age population in recent years. As longer time passed after the initial injury, more associated injuries in the affected knee were found, which may ultimately lead to osteoarthritis. In the ACL-deficient knee with instability, nonoperative treatments ultimately lead to degeneration of the knee and limitation of sports activities. However, radiographic osteoarthritis may still be observed after ACL reconstruction. In this chapter, characteristics and natural histories of ACL injuries with or without ACL reconstruction in different age groups are delineated. While tailoring treatment strategies for the injury, careful evaluation is of great importance to achieve optimal outcomes.