In both hepatitis B and C, host immune responses are responsible for the pathogenesis of the disease as well as for viral control. Although vigorous and broad T cell responses to viral antigens are sufficient for successful control of viral replication or elimination of viruses, many patients have defective immune responses which lead to persistent infection. CD4⁺ T cells are important both for generation of CD8⁺ T cell response and immunological memory. CD8⁺ T cells eliminate the virus by two methods: in one, hepatocytes are destroyed, and in the other, which is noncytolytic, interferon-gamma (IFN-γ) is secreted. Hepatitis B and C viruses both have various mechanisms for escaping host immune responses. CD4⁺CD25⁺ regulatory T cells were recently reported to suppress T cell response, and appear to be involved in insufficient T cell responses in both viral infections. Many studies have shown that host immune responses are involved in effective antiviral treatment. Further understanding of the immune responses is important for developing more effective antiviral therapy for both infections.