Shellfish allergy is one of the most common food allergies, with an estimated prevalence of 3% in the general population. Crustaceans and mollusks are part of this food category, being capable of immunologic hypersensitivity reactions (IgE-mediated, as well as non-IgE-mediated), but also non-immunologic reactions, based on intoxication with certain neurotoxins from contaminated seafood, in which neurological but also gastrointestinal symptoms predominate. The clinical picture in IgE-mediated hypersensitivity reactions can vary in severity, from mild reactions such as oral allergy syndrome to severe reactions such as anaphylaxis. Among the delayed, non-IgE mediated reactions, we can identify food protein-induced enterocolitis syndrome, food protein-induced enteropathy and food protein-induced allergic proctocolitis, which mainly involve gastrointestinal symptoms. Over the past decades, the knowledge gained regarding the characteristics of different allergens has improved the diagnostic approach. Thus, for an accurate diagnosis of seafood-induced allergy, a thorough history, along with skin prick testing and specific IgE dosing are essential, and when these are insufficient, an oral challenge test can disentangle causality.