The genus Equisetum is usually divided into two groups, variously considered as sections, subgenera, or separate genera. The first group, subgenus Equisetum, is characterized by having stomata flush with the epidermis; the species in this group are well-marked and easily distinguished. The second, subgenus, Hippochaete, is characterized by having stomata sunken below the surface of the epidermis; and the species in this group are indistinct and hard to separate. As a result, many different classifications of the Hippochaete have been proposed, and the taxonomy of this group has been confused and in strong need of re-evaluation. This investigation was made to clarify the taxonomic picture of Hippochaete. Approximately six thousand herbarium specimens from all parts of the world were analyzed. In addition, field observations were made, garden and field experiments were conducted, and gametophytes were cultured, to obtain comparative data. The general structure and appearance of mature sporophytes and gametophytes of Hippochaete are described. The nodes of rhizoid formation and sperm discharge are redescribed in the light of new observations, because of the conflicting reports in the previous literature concerning these.