The biological, chemical, and physical factors which are known to influence virus and bacterial survival and transport in the subsurface are discussed. Models used to predict the fate of microorganisms are presented. The models that have been developed to predict the fate of microorganisms do not explicitly address factors observed to influence survival and transport. It has become apparent that it is unlikely that there is a good bacterial indicator of virus behavior, and 'model viruses' such as poliovirus may not mimic the behavior of other viruses such as hapatitis A virus. As methods are developed for the cultivation and detection of other viruses known to cause waterborne disease outbreaks, such as Norwalk and Norwalk-like viruses, these viruses will also have to be evaluated in terms of their ability to survive and be transported in the subsurface environment.