Trees or shrubs, erect to trailing, usually many branched, sometimes forming clumps or mats; trunk, when present, initially segmented, appearing continuous with age, main axis determinate, usually terete. Stem segments green or sometimes reddish to purple, usually flattened, circular, elliptic, ovate, lanceolate, or obovate to oblanceolate, 2–60(–120) × 1.2–40 cm, nearly smooth to tuberculate, glabrous or pubescent; areoles usually elliptic, circular, or obovate, 3–8(–10) × 1–7(–10) mm; wool white, gray, or tan to brown, aging white or gray to black. Spines 0–15+ per areole, white, yellow to brown, red-brown to gray, or black, sometimes partly to wholly white chalky (chalkiness disappearing when wet), aging gray to dark brown to black, with epidermis intact, not sheathed, acicular to subulate, sometimes setose or with hairlike bristles, terete to angular-flattened, to 75(–170) mm, tips sometimes paler or yellow. Glochids in adaxial crescent at margin of areole, in tuft or encircling areole margin, white to yellow to brown, or red-brown, aging white to brown or red-brown. Flowers bisexual or sometimes functionally staminate, radially symmetric; outer tepals green to yellow with margins tinged color of inner tepals; inner tepals pale yellow to orange, pink to red or magenta, rarely white (unicolored) or with base of a different color (bicolored), oblong to spatulate, emarginate-apiculate; nectar chamber simple, open, not covered by proximal thickening style. Pollen yellow, grains reticulate or foveolate (opuntioid type). Fruits sometimes proliferating (sprouting from another fruit), if fleshy, green, yellow, or red to purple or, if dry, tan to gray, straight, sometimes stipitate, clavate to cylindric, ovoid, or obovoid to subspheric, 10–120 × 8–120 mm, fleshy to juicy or dry, smooth or tuberculate, spineless or spiny, sometimes burlike. Seeds pale yellow to tan or gray, generally circular to reniform, flattened (discoid) to subspheric, angular to squarish, sometimes warped, 2–7 × 2–7 mm, glabrous, commonly bearing 1–4 large, shallow depressions due to pressures from adjacent developing seeds; girdle protruding 0.3–3.5 mm, forming ridge or flat wing, or not protruding. x = 11.