There are currently 13 monoclonal antibodies or antibody constructs approved for therapeutic use in the US and at least another 400 products are in clinical testing or undergoing regulatory review. Despite widespread use and development of therapeutic monoclonal antibodies, limited information exists regarding the incidence and consequence of immune antibodies generated against many of these products. In this review, analytical methodology and recent data on the immunogenicity of approved therapeutic monoclonal antibodies will be presented. The implications of these findings in relation to potential concerns for repeated or chronic treatment with these powerful and specific therapeutic agents will be discussed.