Biostability plays an important role in keeping drinking water quality and safety. Methods of biostability assessment are reviewed, including main indicators, traditional analytical methods and emerging ones. Merits and demerits of different analytical methods are discussed respectively. Time-consuming and inaccurate, traditional methods are acknowledged to be inadequate in the evaluation of drinking water quality. Improved methods, e.g. assimilable organic carbon (AOC), total cell concentration (TCC), adenosine tri-phosphate (ATP) methods based on flow cytometry (FCM), are accurate, simple and economical, and thus have shown tremendous potential in water monitoring and engineering practice. Emerging molecular biological methods, e.g. denaturing gradient gel electrophoresis (DGGE) and pyrosequencing, offer an insight into the changes of microbiology compositions in water treatments and distribution, and fundamentally reflect the biological stability of water quality.