To provide an update on recent studies of hepatitis B virus (HBV) and hepatitis C virus (HCV) infections in cancer patients with an emphasis on viral reactivation after cancer treatment, new antiviral therapies, and safety concerns. The diagnostic criteria for HBV reactivation in patients receiving cancer therapy were revised in 2018. HBV reactivation in these patients is preventable, even with the use of new cancer therapies. HCV reactivation also has been reported in cancer patients, particularly those with hematologic malignancies, and is not a virologic condition usually associated with poor outcome. Prophylaxis to prevent HCV reactivation is not recommended because therapy with direct-acting antivirals eradicates the infection in the majority of cancer patients. Cancer patients with HBV or HCV infection are at risk for viral reactivation, with many similarities between these two infections. Patients at high risk for reactivation will benefit significantly from taking oral antivirals, which will reduce the risk of HBV reactivation or prevent development of HCV reactivation following its virologic cure.