Five to 8 genera, about 450 species, nearly worldwide, most diverse in tropical and warm-temperate regions.

The Heliotropiaceae are here treated as a family distinct from the Boraginaceae, in which many botanists traditionally placed them (Steyermark, 1963; Cronquist, 1981, 1991). Species of Heliotropiaceae, are distinguished from the Boraginaceae by a combination of morphological characters, including: a terminal style; fruits either fleshy to more commonly dry and drupelike or less commonly schizocarps, usually separating (sometimes tardily) into 2 or 4 nutlets or mericarps; and the frequent presence of a stigmatic appendage. See the treatment of Boraginaceae (in Volume 2) and Hydrophyllaceae for further discussion. Infrafamilial classification within the Heliotropiaceae is still controversial. Molecular studies (Diane et al., 2002; Hilger and Diane, 2003) have shown that most of the up to eight genera recognized by earlier workers (Förther, 1998) are not natural evolutionary lineages, but have not yet been able to resolve all of the species groups into a comprehensive new classification. The eventual number of genera recognized may be as few as four or as many as six, but most of these will not correspond to the traditionally circumscribed generic groups.