Virtually all therapeutic proteins (biologics) elicit an immune response with the consequent production of anti-drug antibodies (ADA). The majority of ADA to therapeutic monoclonal antibodies (mAbs) are directed against the antigen-binding site of the therapeutic mAb, and hence are neutralizing. This nature of the ADA response explains why fully human antibodies can still be highly immunogenic. The detection of ADA is technically challenging and all assays have limitations, namely a limited capacity in detecting ADA in the presence of a drug due to immune complex (IC) formation, which may underestimate the ADA incidence. Refined assays, able to disrupt drug-ADA ICs, have revealed the presence of ADA in a higher proportion of patients. The great heterogeneity among ADA assays prevents a direct comparison of immunogenicity between different molecules and across studies. The formation of drug-ADA ICs can significantly alter pharmacokinetics and directly reduce drug efficacy if the ADA titer (i.e., concentration) is sufficiently high and persistent. In patients with low ADA titer, free drug concentrations may remain high enough to be effective, while in patients developing high ADA titer a substantial part of the drug will be neutralized and clinical non-response is likely to occur. ADA can also increase the risk of adverse events, namely hypersensitivity reactions. Several studies have revealed the presence of ADA before a clinically overt adverse reaction, highlighting their predictive value. Algorithms integrating therapeutic drug monitoring and immunogenicity information in the current clinical evaluation of patients receiving biologics are today available to guide therapeutic decisions in clinical practice, helping us to design safer and more cost-effective therapeutic strategies.