The introduction of highly purified natural and recombinant single allergenic molecules represented an important improvement in the diagnosis of IgE sensitization. The identification of specific IgE against cross-reacting molecules such as profilin, lipid transfer proteins, calcium binding proteins or against “genuine molecules”, represents an added value and allows to distinguish between true and false polysensitization. In vitro tests add information to recognize patients with sensitization to genuine molecules that cause allergic diseases and to evaluate in childhood the spreading of sensitization for each molecule in order to choose the best treatment and to identify the ideal patient for allergen immunotherapy. Also, in order to detect patients with sensitization to pan-allergens it is important to manage the risk of anaphylaxis for patients allergic to latex and to identify IgE to particular molecules involved in occupational allergy. In patients with negative skin prick tests (SPT), that results in a lower sensitivity compared with in vitro tests, the negative test may be caused by the lack of some important allergenic molecules in the extract used for SPT.