Immunology and microbiology have been linked together since their infancies. The discovery more than 100 years ago that antibodies (Abs) constitute one of the pillars of the body's defense against bacteria and viruses led immediately to the development of serological tests for diagnosis and identification of microorganisms. More recent approaches are based on immunoassay technology, which does not require cross-linking of antigens by Abs. High sensitivity results from the use of labels, such as fluorescent dyes or enzymes, for the detection of antigen binding by Abs. As an example, the quantification of members of the Enterobacteriaceae in drinking water using enzyme immunoassays (Elisa) is presented, which is now available as a DIN standard. Related techniques such as dipstick or dot blot tests originated from the necessity of shortening the analysis time. Immunofluorescence flow cytometry represents the most sophisticated development of this technology to date. A major technological leap is expected from immunosensors, miniaturized measuring devices that selectively detect their targets and provide concentration-dependent signals. When Abs as part of the receptor unit bind their ligands, there is a variation in optical properties, electric charge, mass, or heat, which can be detected directly, ie without tracers, by a variety of transducers. Since sensitivity is directly related to the affinity of the ligand binding, high sensitivity excludes reversibility. Immunochemical methodology is still limited by the availability of selective and sensitive Abs. Future progress will be significantly accelerated by the application of recombinant techniques for Ab production. The main emphasis is directed toward the generation of recombinant Ab libraries, as they are already available for the generation of anti-HIV Ab fragments. It is not surprising therefore that immunochemical methodology, together with PCR-based techniques, belongs to the most promising branch of modern diagnosis.