With a length of 32 to 35 mm, the species is a small, brightly coloured damselfly. It is the bluest of the blue damselflies. Males have a club-shaped black mark on second abdominal segment and the upper surface of segments 8 and 9 is all blue. Males have broad antehumeral stripes, the hind margin of the pronotum has a flat, shallow median lobe. Females occur in three colour forms, typical, blue and brown. They have a medial spine on the underside of segment 8.

In many ways this could be considered to be the most typical British damselfly. It shares its blue and black colouration with several other species. It can be distinguished from the others by its broad ante-humeral stripes. In the male, S2 has a characteristic mark of a spot linked to the inter segment suture by a short line. S8 and S9 are entirely blue.
The female occurs in two colour forms, one blue, as in the male, the other dull green. The mark on S2 is thistle shaped and there is a "Christmas tree" shaped stepped triangle on S8.
They often perch gregariously on emergent plant stems, all facing the same way.

It inhabits a wide range of habitats, from small ponds to rivers. They are especially common at lakes and reservoirs. During mating, the male clasps the female by her neck while she bends her body around to his reproductive organs - this is called a mating wheel. The pair flies together over the water and eggs are laid within a suitable plant, just below the surface. The eggs hatch and the larvae, called nymphs, live in the water and feed on small aquatic animals. Nymphs climb out of the water up a suitable stem to moult into damselflies.

This species is easily confused with the various members of the genus Coenagrion. Look at S2 (on the males) for distinguishing characters. It does appear particularly blue compared with other similar species. The females can be particularly tricky to distinguish apart. The Common Blue Damselfly can be easily mistaken for the Azure Damselfly (Coenagrion puella), but on the back and the thorax, the Common Blue Damselfly has more blue than black; for the Azure Damselfly it is the other way around. The second segment of the thorax has a distinctive spot with a line below connecting to the third segment. Another difference can be observed when inspecting the side of the thorax. The Common Blue Damselfly has only one small black stripe there, while all other blue damselflies have two.

Enallagma cyathigerum is widespread and common in large parts of Europe and northern Asia. In the northern part of its range one of the most common damselflies. There is a possible future threat from habitat destruction and water pollution, as some of it's habitat is not as suitable as it could be, but that does not appear to be having an effect on the global population yet. So it is classified as Least Concern on the IUCN Red List 2006.