This chapter provides some of the new approaches to plant disease diagnosis and pathogen detection that have come about in the last decade as a result of advances in biotechnology. It focuses on nucleic acid hybridization- and antibody-based techniques, and provides a detailed descriptions of these and other methodologies that form the basis of modem plant disease diagnostics. Other antibody-based technologies are available, including radioimmunoassay, immunofluorescence, and immunogold microscopy, but enzyme-linked immunosorbent assay (ELISA) remains the dominant serological technique in plant pathology. Dot immunobinding assays are immunoassays in which the immunochemical reactions are carried out on membranes made of nitrocellulose, nylon, or other materials. Bound antigens are detected by direct or indirect immunoassays using alkaline phosphatase as the enzyme marker. The principle disadvantages of immunofluorescence techniques over ELISA are the absolute requirement for a good-quality microscope with epifluorescence optics, the fact that the assays can be tedious and time consuming, and technical problems resulting from plant and soil autofluorescence.