In brief: Millions of divers, swimmers, and other ocean-goers may encounter jellyfish. If they are stung, they may need either first aid if the case is mild, or extensive treatment if they suffer anaphylaxis, cardiorespiratory problems, or long-term reactions. Some severe cases may even be fatal. The author describes the mechanisms of envenomation and tissue reaction and discusses how to recognize and treat both the immediate and late sequelae.