Infectivity is a fundamental property of viral pathogens such as human rhinoviruses (HRVs). This chapter describes two methods for measuring the infectivity of HRV-A and -B serotypes: end point dilution (TCID50) assay and plaque assay. End point dilution assay is a quantal, not quantitative, assay that determines the dilution of the sample at which 50 % of the aliquots have infectious virus. It can be used for all the HRV-A and -B serotypes and related clinical isolates that grow in cell culture and induce cytopathic effect (CPE), degenerative changes in cells that are visible under a microscope. Plaque assay is a quantitative assay that determines the number of infectious units of a virus in a sample. After an infectious unit of virus infects one cell, the infected cell produces progeny viruses that then infect and kill a circle of adjacent cells. This circle of dead cells detaches from the dish and thus leaves a clear hole in a cell monolayer. Plaque assay works only for HeLa-adapted HRV-A and -B serotypes that can make visible plaques on the cell monolayer. Currently the end point dilution assay and plaque assay have not been developed for the newly discovered HRV-C.