Chronic hepatitis B is a major cause of liver disease worldwide, ranking as the first cause of cirrhosis and hepatocellular carcinoma. Hepatitis B surface antigen (HBsAg) is usually used as a qualitative marker for the diagnosis of hepatitis B virus (HBV) infection. HBsAg clearance is the closest to cure outcome as one can expect to achieve in hepatitis B. Support for this comes from natural history studies demonstrating increased length of survival, lower rates of hepatic decompensation, reduction in the frequency of hepatocellular carcinoma, and regression of liver fibrosis in patients who clear HBsAg. HBsAg seroclearance may occur spontaneously at a yearly incidence of 1-2%, preceded usually by a long period of inactive disease. Interferon treatment enhanced HBsAg seroclearance by approximately three-fold in western studies and sixfold in Asian studies compared with non-treated patients. Pegylated interferon induced a 10-15% yearly rate of HBsAg seroclearance in patients who developed sustained virological response in clinical trials. By contrast, treatment with nucleos (t) ides analogues did not significantly affect the rate of HBsAg seroclearance, especially in patients with hepatitis B e antigen (HBeAg) - negative disease. Recently, serum HBsAg has been shown to be a surrogate marker of covalently closed circular DNA (cccDNA) concentration in the liver. Quantification of serum HBsAg has also been recently shown to be a promising tool for monitoring virologic response in HBeAg-negative patients treated with pegylated interferon.