This issue of ILAR Journal presents an overview of new developments and insights in the study of immunization procedures and adjuvant products. Substantial progress has been made, both with regard to new technologies and to understanding the immune response since publication of an earlier ILAR Journal issue devoted to Adjuvants and Antibody Production (Landi 1995). It is timely and important once again to address these developments. The objective of the current ILAR Journal issue is to discuss critical aspects of immunization in the context of what is known of the immune response and to provide an overview of new developments in the production of polyclonal antibodies (PAbs) and monoclonal antibodies (MAbs). Immunizations in laboratory animals are performed for a wide variety of reasons. Primary purposes include (1) induction of specific B cells for the generation of hybridomas, (2) production of PAbs and MAbs, (3) development and quality control of immunobiological products, (4) fundamental immunological studies, and (5) induction of specific disease models. Clearly, these studies have been integral to scientific breakthroughs that have occurred in many areas of biomedical research. One such critical breakthrough has been the development of highly effective vaccines. In other cases, products such as PAbs and MAbs, which are obtained from immunization procedures, have become essential reagents in the laboratory and are being applied in diagnostic testing, in cancer therapy, and in numerous other ways. Nevertheless, despite the significant medical advances that have resulted, the use of laboratory animals for immunization procedures continues to generate controversy. Investigators and members of animal care and use committees continue to have frequent and fundamental questions related to immunization and adjuvant products. Although the answers to such questions are not always clear and unequivocal, data and information in the field of immunization have increased. Indeed, tradition and myths are gradually being replaced by science-based approaches. For example, descriptions of the adverse reactions of Freund’s complete adjuvant (FCA), which is used for its immunostimulatory capabilities, were first published in 1950 (Leskowitz and Waksman 1950) and then in 1960 (Steiner et al. 1960). Since then, discussions regarding the potential of FCA to induce pain and distress have continued for several decades and have resulted in the preparation of numerous guidelines (e.g., institutional guidelines in the United States, and regulatory guidelines in Canada and several European countries). Currently, the use of FCA is limited in experimental animals, and the process of developing alternative adjuvant products has begun. Concerning the production of MAbs, the Institute for Laboratory Animal Research (ILAR) Committee on Methods of Producing Monoclonal Antibodies concluded in 1999 that the intraperitoneal accumulation of ascites is likely to be associated with pain and distress (NRC 1999). Committee members recommended that the tissue-culture method for the production of MAbs should be adopted as the routine method unless there is clear reason why tissue culture methods cannot be used or why their use would represent an unreasonable barrier to obtaining the product at a cost consistent with the realities of funding biomedical research programs. In several European countries, the use of the ascites method is strongly discouraged, although it has not been banned entirely. As new technologies and new experimental data become available, we are gradually unraveling the complex processes underlying immunization procedures. This information will be helpful in understanding the critical aspects involved in immunization protocols and will allow us to prepare rationally based guidelines on how to design optimal immunization protocols and how to continue reducing, refining, and replacing the use of animals. In their article, McCullough and Summerfield (2005) introduce the basic concepts of the immune responses. The Coenraad F. M. Hendriksen, D.V.M., Ph.D., is an Animal Welfare Officer and Senior Scientist at the Netherlands Vaccine Institute, Bilthoven, The Netherlands.