About 14 genera, 200–260 species, North America to South America, Asia, Africa, mostly in temperate and tropical America.

Ovule morphology, the presence of iridoid compounds (Kooiman, 1974), and molecular data (Hempel et al., 1995) indicate that the Loasaceae are closely related to the Hydrangeaceae and other members of the Cornales. Members of the family are distinguished by their brittle stems and variety of specialized trichomes. Species in some genera, including Mentzelia, have remarkable barbed hairs that stick tightly to clothing and skin (even surviving trips to the laundromat), giving rise to such common names as stickleaf and velcro plant. Some other genera, like the closely related Eucnide Zucc., instead have stinging hairs, and the family also includes members with calcified hairs and various glandular trichomes (Ernst and Thompson, 1963).