Allergic reactions to fish are potentially life-threatening. We detail here various reactions to fish, notably anaphylaxis and food protein induced enterocolitis syndrome, as well as confounders such as reactions to the parasite Anisakis and scombroid poisoning. Most IgE-mediated reactions are directed to β-parvalbumin. This thermo-resistant protein is highly conserved between species of osseous fish, and accounts for allergic cross-reactivity. Cartilaginous fish such as rays express α-parvalbumin, and may constitute an alternative food source in subjects allergic to the beta-form. Diagnosis relies on a meticulous history, skin-prick testing preferably with fresh fish; search for specific IgE and food challenge. Once the diagnosis is established, prevention consists in avoiding all fish products susceptible to induce a reaction.