Two methods are described for inducing Giardia lamblia to excyst. Both at times can be shown to yield trophozoites, although the success rate varies widely depending on the investigator and the cyst source. The excystation procedure for Giardia muris is much more satisfactory; however, results using it as a substitute or model for Giardia lamblia should be interpreted with great caution. It is suggested that all the factors involved in excystation have not been identified as yet. Several areas that appear to need rapid attention are the composition of the cyst wall, definition of factors in the incubation media that promote excystation, and determination of the presence or absence of endogenous enzymes associated with Giardia trophozoites. Also a study of the relationship of excystation to infectivity is needed badly. Presently excystation is only a measure of viability. The investigator interested in studying Giardia excystation should use these methods as a starting point for delineating the process even further.