It is well known that interferons (IFNs) have antiviral activity (1,2). In fact, the definition of interferon springs from this observation. Although it is reported that other cytokines, e.g., tumor necrosis factor (TNF) (3), may exert antiviral effects, biological assays based on antiviral activity, or antiviral assays (AVA), were originally developed solely for potency estimations of IFNs (4,5). The development of AVA started more than 40 yr ago and has continued and progressed with the discovery and characterization of IFN. Although many IFNs are now known to have multiple activities, e.g., antiviral, antiproliferative, immunostimulatory (1,2), the AVA remains one of the most widely used methods of measuring IFN potency (4,5). This chapter is therefore devoted to describing the methodology underlying AVAs (see Subheading 3.2.), and includes tips on how to best carry these out in the laboratory.