Plants perennial herbs (a few species annual). Stems erect to loosely ascending from a spreading base, with rhizomes (except in annual species, which are taprooted), sometimes forming dense mats or low mounds (to cushion-like elsewhere), unbranched or branched (at the base or toward the tip). Leaves opposite, the uppermost sometimes subopposite or rarely alternate, sessile to subsessile, simple, unlobed, the margins entire, the bases of the opposing leaves sometimes forming a narrow ridge or membrane across the node. Inflorescences terminal and often also axillary from the upper leaves, consisting of clusters or small panicles, sometimes appearing narrowly racemose or reduced to a solitary flower. Calyces 5-lobed to above or near the midpoint (in some species some of the lobes separating tardily after the flower opens), tubular to narrowly bell-shaped, differentiated into 5 thicker, green bands (extending into the lobes), these separated by intervening thin, translucent areas (these delicate and often rupturing as the fruits mature), glabrous to densely pubescent with short, glandular and/or nonglandular, spreading hairs. Corollas trumpet-shaped, white to more commonly pink, purple, or occasionally blue (sometimes red in P. drummondii; yellow elsewhere), often with lighter or darker markings at the mouth of the tube and lobe bases, the tube slender, somewhat expanded near the tip, the lobes abruptly spreading, rounded or angled to a broad, blunt tip, sometimes abruptly tapered to a minute, sharply pointed extension, in a few species coarsely notched apically. Stamens with the anthers positioned unequally along the tube, either all included or some of them positioned near or less commonly slightly exerted from the mouth of the tube. Style either short and not extending past the midpoint of the corolla tube or longer and extending to near the mouth of the corolla tube. Seeds 3 (rarely 1–12 elsewhere), 2–4 mm long, oblong-ovate to occasionally broadly ellipsoid, slightly flattened, the surface yellowish brown to dark brown, faintly to moderately wrinkled, not becoming sticky when moistened. About 60 species, North America, Asia (Siberia), one species (P. drummondii) introduced sporadically nearly worldwide.

Phlox is the largest genus of the Polemoniaceae, and exhibits interesting diversity relative to ecology and geography across much of North America. Approximately a third of the species occur in eastern North America, and the genus is well-represented in the Missouri flora. Phlox is known for hybridization and polyploidy, and these factors have likely contributed to a complicated phylogenetic history (C. J. Ferguson and Jansen 2002). Most of the species in our flora, however, are readily distinguished, and an encounter with a hybrid in the field would be very rare. A valuable recent work on the genus, including horticultural information, was provided by Locklear (2011).