Biologic therapies, predominantly TNF-α inhibitors, have revolutionized the treatment of rheumatoid arthritis (RA). However, their clinical utility can be limited by the development of antidrug antibodies (ADAs). Immunogenicity is a complex phenomenon related to various drug, disease, and patient characteristics, and may be more common with the monoclonal antibodies than with etanercept, a soluble TNF receptor-Fc immunoglobulin fusion protein. Neutralizing antibodies - those that hinder bioactivity by preventing drug molecules from binding to TNF - are correlated with reduced serum drug concentrations, loss of therapeutic response, adverse events, and treatment discontinuation. Cost-effective use of these agents will depend on further research into drug and ADA assays, and how they should guide dose reduction or switching strategies.