Occult hepatitis B (OHB), or persistent hepatitis B virus (HBV) viremia in surface-antigen-HBsAg-negative patients, has been recognized as a medical concern during the last decade. The exact magnitude, pathogenesis and clinical relevance of OHB are unclear. This review organizes the published data on OHB and presents an overview of the current hypotheses on OHB's pathogenesis and clinical relevance.Many explanations have been offered for the pathogenesis of OHB, ranging from the inability of standard immunoassays to diagnose OHB to the involvement of the versatile virus-host factors. Also, special care should be taken regarding the diagnosis of OBH. It seems that both shared viral-host factors are involved in the pathogenesis of OBH. Further molecular studies on cohort patients group need to explore such association.