Anisakis simplex is a parasite, belonging to the Anisakidae family. The life cycle of the parasite can include one or more intermediary hosts, their final hosts being marine mammals or large fish, in which the larvae develop until the adult stage is reached. Man is an accidental host who acquires the larvae by eating raw or undercooked fish. Since the mid-50s, when the first case studies were published in Holland and Japan, new cases have been emerging in different countries including Spain. Parasitization of man by the living larva is known as anisakiasis, principally giving rise to digestive symptomology, with other rare cases described of invasion of other organs such as the lung, the liver, the spleen, the pancreas, etc. Clinical pictures of allergy to IgE mediated anisakis simplex have also been described: reactions by thermostable antigens of the parasite that develop in spite of the fish being cooked or frozen, and an acute digestive parasitization with allergic symptoms called gastro-allergic anisakiasis. In the diagnosis of anisakiasis and/or allergy to Anisakis, the antecedent of the prior ingestion of fish as well as the clinical accompaniment can form basic data of considerable orientational value, and endoscopy can reveal the presence of the larvae and make possible their extraction. Besides, in cases of allergy the detection test for specific IgE facing Anisakis simplex, and cutaneous tests with fish should be carried out. The best treatment for avoiding this parasitization is prophylactic, avoiding the consumption of raw or undercooked fish, while a fish free diet is necessary in cases of true allergy to the thermostable proteins of the parasite.