IgE antibody analyses using serological methods are an integral part of the diagnostic evaluation of a patient for allergic disease. They serve to clarify whether a state of sensitization exists in the patient as one of the many risk factors for elicitation of allergic symptoms. This overview examines the role that IgE antibody measurements play in the diagnostic algorithm when considering the pretest likelihood of disease on the basis of the patient's clinical history. Each of the 4 allergen groups (inhalants, venoms, drugs, and foods) are discussed in the context of the various in vitro and in vivo modalities for evaluating sensitization to allergens. Both the past and present analytical methods for IgE antibody detection and quantification in serum are critiqued. Causes for discordant IgE antibody levels with allergy symptoms are discussed with a special focus on analytically valid but clinically irrelevant positive IgE responses. Finally, applications are discussed where allergenic molecules provide enhanced analytical and diagnostic sensitivity and specificity when compared with results generated with allergen extract-based IgE assays.