Allergic cross-reactivity is an important epidemiological issue in all age groups. It is caused by a non-specific binding of both primary allergen as well as allergens causing secondary cross-reactivity by the same IgE antibodies. This phenomenon results from the similarity of the molecular structure of allergen epitopes and leads to a number of allergic cross-reactivity syndromes, such as pollen-food syndromes, pork-cat syndrome or latex-fruit syndrome. They are characterized by rich symptomatology and the possible occurrence of symptoms related to various systems, including life-threatening systemic reactions. In many cases, specific allergen groups responsible for certain cross-reactions, such as plant profilins, fish parvalbumins or invertebrate tropomyosins, have been identified. Also, some of the factors affecting the spatial conformation of allergens, and thus modifying their allergenic potential, have been identified. Despite all these achievements, the diagnostics of cross reactivity syndromes still remains difficult due to the limited available methods and the possible occurrence of overlapping phenomena such as co-sensitisation, asymptomatic cross-sensitisation or IgE-independent or nonimmunological adverse drug reactions. Therefore, careful management based on medical history as well as avoidance of unjustified treatment methods, e.g. diet therapy or immunotherapy, are necessary. This is of great importance as the incidence of food allergies is expected to increase mainly due to the progressive rise in the prevalence of inhalant allergies to pollens.