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All folia demonstrate a few common elements: folia (singular, folium) are planar or sub-planar speleothems, slanting downward and away from cave walls and ceilings, and genetically associated with fluctuating environmental interfaces. They were first identified and named from examples in western USA hypogenic caves but have since been identified elsewhere in the world. Folia shelves are typically composed of calcite and interweave smoothly along sub-horizontal contours. Calcite accretion, both chemical and mechanical from suspended particles, follows water-surface fluctuations to form protrusions in arrays along walls and ceilings where new accretion tends to follow the water-surface downward during descending phases, gradually developing into slanted ridges with slopes <~45º. Some workers limit the term to calcite forms, but we recognize six folia types according to the controlling interface: 1.) Common calcite folia form at the interface of calcite-supersaturated water and air and are here referred to as “interweaving calcite folia”; 2.) “Hells Bells” form at the interface of a saltwater/sulfidic-freshwater halocline/redoxcline; 3.) Sulfur folia form at the interface of hydrogen-sulfide-enriched air and oxygenated air; 4.) Mud folia form at the interface of muddy water and air and may include cavity collars outlining bubble pockets; 5.) Halite folia form at the interface of saltwater and air; and 6.) Lava folia form at the interface of molten lava and volcanic gas. Two additional features have been proposed as possible folia but are rejected as folia types in this paper. The first is a non-speleothem “flange” feature formed by fluid venting into oceanic or saline-lake waters. The second is a rare speleothem called “cave leaves” that apparently form on the ceilings of drowned passages where glacial melt infiltrates and mixes with the cave-stream water.