Heterotheca inuloides (Asteraceae) is commonly called in Spanish árnica or Mexican árnica because of its resemblance to flowers to the European arnica (Arnica montana) and is the most important species of the “arnica complex” sold at the Sonora Market (an important national market of medicinal plants in Mexico City). This medicinal plant complex includes, among other species: Arnica montana, Galphimia glauca, Chrysactinia mexicana, Oenothera rosea, and Cuphea aequipetala. To supply the Sonora Market, people collected wild plants of H. inuloides var. inuloides in the southeastern region of the state of Mexico up to the 1990s. Later, due to the increased demand for this species, farmers of this region began to cultivate it on a large scale. Since then, the arnica grown and commercialized in the region exhibit characteristics of other varieties (var. rosei, var. viridis, and var. leptoglossa), possibly due to the introduction of seeds from neighboring populations derived from other markets. Flowers, stems, and leaves of H. inuloides are used for treating inflammatory processes, bruises, muscular pains, rheumatic pain, and stomachache; it is usually drunk as infusions and applied topically. The wide variety of chemical constituents isolated from H. inuloides include polyacetylene, sesquiterpenes (mainly cadinanoids), triterpenes, phytosterols, and flavonoids, among other compounds. Biological evaluations of these extracts and purified substances reveal their antioxidants, cytotoxic, anti-inflammatory, antimicrobial, phytotoxic, and insecticidal properties. The principal bioactivity can be attributed to cadinanoid sesquiterpenes and flavonoids. Molecular markers (DNA barcodes) applied to commercialized pure and mixed vegetal arnica powders can differentiate the signature species, H. inuloides, from the other members of the arnica complex. The most useful molecular markers are rbcL and ITS2.