Strictly speaking, biological drugs are defined as drugs obtained using biotechnology that act on the immune system. They encompass monoclonal antibodies, fusion proteins, and cytokines. Although they are restricted to specific diseases, they have been increasingly used in recent years, with the consequent reporting of adverse reactions, many of which occur during the postmarketing phase. Because of the characteristics of adverse reactions, a new classification has been proposed. Hypersensitivity reactions are beta-type reactions and include infusion reactions and injection site reactions. In some cases, an immune mechanism mediated by IgE, IgG, or T cells is involved. Clinical symptoms vary widely, from skin reactions to anaphylaxis. Diagnostic studies are based on skin tests and in vitro tests (specific IgE, basophil activation test). Most are not standardized and are conducted in small groups of patients, thus making it impossible to obtain sensitivity and specificity values. With some biological drugs, desensitization protocols have proven successful. In this review, we discuss hypersensitivity reactions to biological drugs and the diagnostic tests used to assess these reactions.