The Hydrophilinae is a large and cosmopolitan group of more than 1600 species divided among 6 tribes, 5 of which are represented in the U.K. All are aquatic insects typified by the various groups constituting the U.K. fauna. Size varies widely, 1-50mm, although most are <10mm. Many superficially resemble the Dytiscidae with a streamlined appearance; oval and more or less smoothly continuous in outline, convex above but, unlike the dytiscids, flat below. The upperside is glabrous. Head usually prominent with the eyes either convex and protruding e.g. in Berosus species, or smoothly continuous with the outline e.g. Enochrus species. A feature characterizing the group is the palps which are as long as, and often longer than, the antennae. The antennae are 7-9 segmented with a 3-segmented and pubescent club. The pronotum is smooth and variously punctured, lacks depressions and is broadest at the base. The elytra are smooth, rounded in outline and glabrous although the punctures may bear setae or bristles, and variously punctured and striate. The legs sometimes have long swimming hairs. Tarsi generally 5-5-5; in Cymbiodyta species 5-4-4 and Berosus species are dimorphic; 5-5-5 in the female and 4-5-5 in the male. The only non-British tribe, the Sperchopsini Hansen, 1991, includes about 25 Asian and Nearctic species.

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Many species live in marginal situations, especially in well vegetated areas and even more so where this is patchy; marshes , ponds and slow-moving water, shallow environments are preferred because of the respiratory behaviour. In the absence of permanent water bodies many species can be found in temporary pools. Because many species inhabit shallow waters they can be heavily affected by avian and fish predation; in seasonal wetlands in California they are an important food source for waterfowl. All species swim with alternate movements of the mid- and hind-legs and renew their air supply head first (unlike dytiscids) at the water surface, in most cases the antennae being modified in some way to facilitate this. In many cases the adults are long lived, 2-3 years, and produce a single generation each year. Mating and oviposition generally occur in water; the females of some species carry the eggs beneath the abdomen but most enclose them in a densely woven silk cocoon which has one end elongated into a ‘mast’ which helps with respiration. Cocoons are usually attached to emergent vegetation just above the water or in marginal areas near the water’s edge. The eggs generally hatch within a week or two. Larvae are usually entirely carnivorous and spend most of their time searching for prey; they consume a wide range of organisms although some are specialist feeders and those of Helochares Mulsant, 1844 are herbivores.  They are free swimming and have well-developed and serrated mandibles, sometimes prey is taken out of the water to immobilize it. Most breath atmospheric oxygen through spiracles on the apex of the abdomen but some have also developed gills e.g. Berosus species have thoracic gills which allow them to remain submerged longer and live in deeper water. Pupation occurs out of the water, generally among marginal vegetation or in cells below the soil surface. Adults of most species can fly and will often disperse by flight after eclosion. They are mostly vegetarian but will occasionally take invertebrates or scavenge carrion etc.

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Hydrophilinae are distinctive; they lack the pronotal grooves of the Helophoridae and the base of the pronotum is as wide as that of the elytra so separating them from the Hydrochidae and the Georissidae. Some, or even many, superficially resemble dytiscids but are distinguished by, among other things, the form of the palps and the antennae. The only tribe not represented in Europe is the Sperchopsini Hansen, 1991 which contains 5 genera and about 25 species. They occur in North America, Canada, Alaska and Asia. The following few points relating to the various tribes are intended to put the U.K. fauna into some context but it must be remembered that a familiarity of our fauna will allow the majority of the world fauna to be recognized and appreciated. Classification is not settled and many other systems can be found, more especially as the group seems to be the subject of extensive worldwide study.

• Berosini Mulsant, 1844 A large group with about 350 species in 5 genera. Cosmopolitan with many Neotropical species. The genus Berosus Leach, 1817 includes >260 species worldwide with 17 from North America and 6 from Canada. All are aquatic and very distinct species.

• Chaetarthriini Bedel, 1881 contains both Holarctic and tropical species. Includes the U.K. genus Chaetarthria Stephens, 1833; 2 U.K. species, total >40. Aquatic or marginal species, adults are often found buried in the substrate. Oval and convex with the first abdominal sternite lined with long golden pubescence which covers the second sternite.

• Anacaenini Hansen, 1991 is a cosmopolitan group containing >250 species. The genus Anacaena Thomson, 1859 contains about 110 species of which only 4 are Nearctic including 2 of the U.K. species, A. limbata (Fabricius, 1792) and A. lutescens (Stephens, 1829).

• Laccobiini Bertrand, 1954 species are distinctive; small, up to 5mm with numerous lines of punctures on the elytra and without spines on the metasternum. Palps robust and about as long as the antennae, last segment longer than the penultimate. 12 genera. Laccobius Erichson, 1837 is a widespread genus of about 250 species included in 10 subgenera, 8 species are British and many occur in Central America. The genus Oocyclus Sharp, 1882 includes a single Central American species.

• Hydrophilini Latreille, 1802 is sometimes divided into 2 tribes; the hydrophilini and the Acidocerini Zaitzev, 1908. This tribe contains up to 22 genera worldwide including several that occur in the U.K.  Enochrus Thomson, 1859 is the third largest genus of the family with about 220 species in 6 subgenera worldwide, Cymbiodyta Bedel, 1881 contains 31 Palaearctic and Nearctic species, and Helochares Mulsant, 1884 is cosmopolitan with>180 species in 5 subgenera. Larger species are included in the genera Hydrochara Berthold, 1827 with 23 Holarctic and African species, and Hydrophilus Geoffroy, 1762 with about 50 species in 3 subgenera.