Hepatitis B is a liver inflammation caused by hepatitis B virus (HBV) infection. Usually it occurs as acute hepatitis, but in some cases virus remains in patient, causing a chronic infection. Consequently, it can lead to development of cirrhosis and hepatocellular carcinoma. Worldwide, there are an estimated 240 million chronically infected persons. Therefore, chronic HBV infection is a major public health issue. Hepatitis B virus is very infectious and it spreads through contact with infected body fluids. Virus replication includes reverse transcription using a viral DNA polymerase lacking proof-reading ability. This results in the emergence of mutant viruses that can be selected out by host immunity or antiviral drugs. Numerous research studies have shown that the speed of disease progression, reliability of diagnostic methods and the success of antiviral therapy and immunization are all influenced by genetic variability of the virus. Mutations in hepatitis B surface antigen (HBsAg) may lead to evasion of vaccine-induced immunity („vaccine escape“ mutants) and loss of HBV detection by diagnostic assays. The inability to detect HBV means that these individuals may become blood donors and continue HBV transmission. Also, mutations in YMDD motif of viral DNA polimerase are associated with resistance to nucleos(t)ide analogues. Monitoring of HBV mutations contributes to improvement of prevention, diagnosis and creating more efficient antiviral therapy.