Foreign bodies are uncommon, but they are important and interesting. Foreign bodies may be ingested, inserted into a body cavity, or deposited into the body by a traumatic or iatrogenic injury. Most ingested foreign bodies pass through the gastrointestinal tract without a problem. Most foreign bodies inserted into a body cavity cause only minor mucosal injury. However, ingested or inserted foreign bodies may cause bowel obstruction or perforation; lead to severe hemorrhage, abscess formation, or septicemia; or undergo distant embolization. Motor vehicle accidents and bullet wounds are common causes of traumatic foreign bodies. Metallic objects, except aluminum, are opaque, and most animal bones and all glass foreign bodies are opaque on radiographs. Most plastic and wooden foreign bodies (cactus thorns, splinters) and most fish bones are not opaque on radiographs. All patients should be thoroughly screened for foreign bodies before undergoing a magnetic resonance imaging study.