The human respiratory tract virome is defined here as the viruses present in the human respiratory tract that can infect human cells. Sensitive, culture-independent molecular assays (polymerase chain reaction and high-throughput sequencing) reveal that in addition to common viruses that cause acute, symptomatic infections the virome also includes viruses that do not cause clinical symptoms, have unknown pathogenic effect, or cause symptoms but are not among the most common viral respiratory tract pathogens. These molecular tools provide means for better defining the virome and studying the effects of viral infections on the dynamics of chronic lung diseases.