Food allergy is an increasing and potentially serious problem that can significantly limit patient’s quality of life. The incidence of this disease has been augmenting in recent decades. Classic allergy tests do not reflect the exact immunological reality of allergy. Recently, many food allergens have been characterized at the molecular level, allowing an increased understanding of the immunopathogeny of many allergic disorders and also the development of molecular diagnosis in this field. The use of allergen components and the correct interpretation of the tests results require some degree of knowledge about the basis of allergen components, the concept of cross-reactivity Review Article Valério et al.; JAMPS, 3(2): 52-60, 2015; Article no.JAMPS.2015.026 53 and their clinical implications. In severe cases the study of molecular components of food allergens allows to define the allergenic phenotype of each patient. It also allows a more accurate evaluation of cross-reactivity that is defined as the immunological recognition by an IgE antibody of similar allergenic molecules (homologues) present in different food, improving patients follow up, with a more appropriate therapeutic decision of each case. One of the most innovating tests in this area is ImmunoCAP ISAC, which evaluates the patient’s reactivity to several allergens in a single assay, using a small sample of serum. It is a very useful tool to establish a well-defined diagnostic in more complex cases. However, it is important to remember that the costs resulting from the use of molecular diagnosis tests are not negligible and that they should be considered after conducting a thorough clinical story as first line approach and after second line diagnostic exams like skin prick tests and other less expensive specific IgE tests.