Immunoglobulins occur as monomers and possess a pair of identical heavy and light chains. Unless enzymatically degraded, each monomeric antibody molecule has two identical combining sites. In humans and many common experimental animals, two light chain isotypes, namely, kappa and lambda, and five heavy chain class isotypes, namely, Immunoglobulins gamma, Immunoglobulins mu, Immunoglobulins alpha, Immunoglobulins delta, and Immunoglobulins epsilon, have been identified. While the constant region of an antibody molecule is not involved in the combination with antigen, this region is important in the discussion of antibody-antigen reactions. The chapter concentrates on primary binding and competition assays, the precipitin reaction as an example of a secondary reaction with only cursory mention of other common secondary serological assays, and multiple antibody binding assays. Karl Landsteiner showed that simple aromatic compounds, when coupled to a "Schlepper" or carrier protein, could induce the production of antibodies, some of which were specific for the attached compound or haptens.