Phacus is a genus of unicellular euglenoids, of the phylum Euglenozoa (also known as Euglenophyta), characterized by its flat, leaf-shaped structure, and rigid cytoskeleton known as a pellicle. These eukaryotes are mostly green in colour, and have a single flagellum that extends the length of their body. They are morphologically very flat, rigid, leaf-shaped, and contain many small discoid chloroplasts.

Phacus are commonly found in freshwater habitats around the globe and include several hundred species that continue to be discovered to this day. There are 564 species of Phacus in the database, but only 171 have been accepted taxonomically. It is a large and complex genus, with ongoing species revisions continuing to the present.

The genus name is believed to have originated from the Greek word φακός (phakós), meaning lentil or lens. This may be due to the general round or oval shape of the many species that are part of this genus. Its origins date back to the nineteenth century, in Germany, where it was first coined by Nitzsch and later formally established by Dujardin. The genus name is treated in literature as masculine.

Otto Friedrich Müller is credited to have first seen organisms matching Phacus, in 1773; he named his organism Cercaria pleuronectes and placed it in the genus Cercaria. However, Christian Ludwig Nitzsch in 1817 noted that his genus Cercaria was an artificial grouping of different organisms, and later in 1827 transferred the genus Cercaria pleuronectes to the genus Phacus. Earlier in 1823, Jean-Baptiste Bory de Saint-Vincent proposed the name Virgulina which technically had priority over the name Phacus. The name Virgulina was later rejected in favor of Phacus.

The genus in its modern sense was established by the French biologist Félix Dujardin in 1841, using Nitzsch's name. Dujardin first published collective findings of the genus in the scientific journal Histoire naturelle des Zoophytes, Infusoires in 1841, separating them from the genus Euglena. The reason behind the separation was in order to create a group that correctly organizes their established morphological characteristics such as their rigid, flat, leaf-shape and small discoid chloroplasts with absent pyrenoids.

Christian Ehrenberg was one of the first scientists to discover and classify members of the genus; his discovery of Phacus longicauda (Ehrenberg) was one of the first species of the genus to be discovered (1830) and was later used as the lectotype species for the genus. Laters workers such as Lemmermann (1930), Pochmann (1942) and Huber-Pestalozzi (1955) added additional species. They also attempted to subdivide Phacus into a number of subgenera and sections, mostly based on the morphology of the spiral pellicle surrounding the cells.

Since the establishment of the genus, classification of taxa has been difficult because many species display enormous morphological variability in characters such as cell shape and size. Many taxa of different ranks (species, varieties and forms) have been described, but the criteria for distinguishing them are often vague. With the advent of molecular phylogenetic methods, Phacus has undergone significant reclassification and some species are now distinguished by more reliable morphological differences, such as the presence of perpendicular "struts" in the periplast. Some species have been found to be cryptic, and are difficult or impossible to distinguish from each other using morphology alone. Two species, Euglena limnophila and Lepocinclis salinus, were found to be nested within the clade of Phacus. The genus Phacus was therefore redefined to include these species, preserving its monophyly, but these two species (now classified as Phacus limnophilus and Phacus salinus) differ from other Phacus in not having flattened cells.

Phacus are commonly found in freshwater habitats all over the world. Many species of this genus have been discovered in several countries, including Japan, the United States, Portugal, Brazil, Korea and the Philippines. Different members of the genus have been found in temperatures ranging from 11.4 to 21.6 °C, and a pH between 6.2 and 7.5. Phacus organisms are found in a range of freshwater environments (some more acidic or alkaline than others), prefer cooler temperatures, and on average exist in more neutral pH aquatic habitats. Many species of Phacus are considered to be euplanktonic (free-floating organisms or open water plankton) because they are commonly found together with other genera of euglenids such as Lepocinclis, Trachelomonas, Euglena. Although very common, they rarely become numerous enough to form blooms, unlike Euglena and Trachelomonas.