Laboratorians continually seek methodologies that yield accurate results in a timely fashion, are cost effective, and require less technical expertise. Diagnosis of viral infections via viral antigen detection methods such as immunofluorescence (FA), immunochromatography (lateral flow) (IC), and enzyme immunoassays (EIA) offer many of these attractive features and are useful for direct detection of viral antigens in an array of clinical specimens and for identification of cultivated viruses. Whether the detection method is FA, rapid IC, or EIA, detection of antigens of the common respiratory viruses (i.e., adenovirus; influenza virus [Flu] A and B; parainfluenza virus [PIV] −1, −2, and −3 and respiratory syncytial virus [RSV]), has been shown to be more useful in patient management than either traditional virus isolation (1, 2, 3) or viral detection in rapid culture using centrifugation-enhanced inoculation (4). There is considerable variability in the sensitivity, specificity, technical considerations, and turnaround time among the various methods, and each method may perform differently depending on the viral target. This chapter deals with principles of FA, IC, and EIA and their contemporary applications in viral antigen detection.