Muscle strain is a very common injury. Muscles that are frequently involved cross two joints, act mainly in an eccentric fashion, and contain a high percentage of fast-twitch fibers. Muscle strain usually causes acute pain and occurs during strenuous activity. In most cases, the diagnosis can be made on the basis of the history and physical examination. Magnetic resonance imaging is recommended only when radiologic evaluation is necessary for diagnosis. Initial treatment consists of rest, ice, compression, and nonsteroidal antiinflammatory drug therapy. As pain and swelling subside, physical therapy should be initiated to restore flexibility and strength. Avoiding excessive fatigue and performing adequate warm-up before intense exercise may help to prevent muscle strain injury. The long-term outcome after muscle strain injury is usually excellent, and complications are few.