Germinal center responses are the mechanism that the immune system uses normally to generate high affinity antigen-specific B-cell receptors and secreted immunoglobulins. Genetically altered mice have provided powerful tools for dissecting the physiology of these germinal center responses. In this review, we have attempted to summarize information from various sources and interpret the new observations based on what was previously known. A section is included to review the basic anatomy of the relevant structures in lymph node and spleen. A summary of the mutant mice producing a phenotype where germinal center responses are altered is also furnished. This review is aimed at providing useful information to those working in this field as well as those wishing to understand more about in vivo immunology.