Aquatic or terrestrial, perennial, heterosporous herbs. Stems 1, rarely 2 rings of meristematic cells producing secondary tissue, short and stout, dichotomously branched, the roots arising from the 2- or 3-lobed stem base. Leaves crowded in a dense rosette, subulate or filiform, usually more or less terete, often tubular and septate, sheathing at base; with a minute ligule on the upper side near the base. The first-produced leaves in any season bearing megasporangia, the next microsporangia, and the last sterile. Sporangia sessile, more or less embedded in the leafbase below the ligule and usually covered by an indusium formed from the leaf-base. Spores on germination giving rise to prothalli. Male gametophytes (from microspore) of one vegetative cell and an antheridium, with a 4-celled wall surrounding 2 cells which give rise to 4 spermatozoids. The multiciliate spermatozoids are liberated by the dehiscence of the spore and breaking down of the antheridium wall. Female gametophyte (from megaspore) is manycelled, fills the megaspore and bears archegonia, the necks of which protrude from the split top of the megaspore. The young plant develops with a resting stage from the fertilised archegonium.

Contains 2 genera, Isoetes and the monotypic Stylites from the Andes of Peru, sometimes only recognised as a subgenus.