Jellyfish venoms are mixtures of toxic and/or antigenic polypeptides and enzymes pathogenic to human beings. As newer therapeutic agents become available to treat the various reactions to stings caused by these animals, an accurate diagnosis of the type of reaction the patient experiences and of the offending species will be necessary. Fatal reactions may be caused either by anaphylaxis or by the action of toxins in the venom on the heart, respiratory center, or kidneys. Cutaneous eruptions after envenomation may be local, generalized, exaggerated, recurrent, delayed, persistent, or occur at sites distant from the primary sting. Fat atrophy, pigmentary changes, vasospasm, and contractures with gangrene can occur after jellyfish stings. Identification of the envenoming animal can be made by actual visualization, examination for nematocysts on skin scraping, or serologically. It may also be predicted based on knowledge of location, time, and environmental circumstances of the encounter. First-aid measures designed to prevent additional nematocyst rupture are species-specific. Anaphylaxis should be prevented by the appropriate lifesaving measures. Other syndromes, caused by the toxins of the venom or mediated by humoral or cellular immune mechanisms, should be treated specifically.