Over the past 55 years a number of methods have been described for excysting Giardia cysts as a means of determining viability. The excystation methods for G. muris cysts are reliable and reproducible. However, methods published to date for the excystation of G. lamblia cysts, the human pathogen, have not yielded reliable, reproducible results. For both Giardia cyst types the stimulatory factors promoting excystation are low pH, carbon dioxide, a temperature around 37 C, and a final neutralizing step at pH 7.0. Recently, other methods have been developed for determining Giardia cyst viability. These include several fluorescent vital dyes, differential interference contrast microscopy, and Mongolian gerbil infectivity tests.