In older classifications it used to be the only family of the order Plantaginales, but numerous phylogenetic studies, summarized by the Angiosperm Phylogeny Group, have demonstrated that this taxon should be included within the Lamiales.
The plantain family as traditionally circumscribed consisted of only three genera, Bougueria, Littorella, and Plantago. However, new phylogenetic research has indicated that Plantaginaceae s.s. (s.s. = sensu stricto, in the strict sense) is nested within several genera previously included in Scrophulariaceae (but not including the type genus, Scrophularia). Although Veronicaceae (1782) is the oldest family name for this group, Plantaginaceae (1789) is a conserved name under the International Code of Botanical Nomenclature (ICBN) and thus has priority over any earlier family name within the circumscription of any group containing Plantaginaceae. Furthermore, the ICBN does not consider family names published before 1789 to be names eligible for conservation, thus ruling out Veronicaceae. The name Antirrhinaceae has been proposed for conservation over Plantaginaceae. In the meantime, the Angiosperm Phylogeny Group has accepted the name Plantaginaceae. However, Olmstead (2003) has chosen to use the name Veronicaceae.
The Plantaginaceae s.l. (s.l. = sensu lato, in the broad sense) is a most diverse, cosmopolitan family, occurring mostly in temperate zones. It consists of herbs, shrubs and also a few aquatic plants with roots (such as the genus Callitriche). Being so diverse, the circumscription of this family is difficult to establish.
The structure and form of the flowers can be very variable. Some genera are 4-merous (i.e. with 4 sepals and 4 petals), such as Aragoa (but this one has 5 sepals); others are 5-8-merous, such as Sibthorpia. The flowers of most genera are polysymmetric. The corolla is often two-lipped. In some taxa, the androecium is formed before the corolla.
The fruit is a capsule that dehisces through the partitions between the cells. In Veronica this partition is in the length; in species of Antirrhineae the dehiscence releases the pollen through the pores at the tip of the anther; or it may come about through a transverse circular line around the capsule.