The male has translucent wings which each have a broad, dark iridescent blue-black spot (or band) across the outer part. On immature dragonflies the spot is dark brown. The body can be a metallic blue or blue-ish green.

The female has translucent, pale green iridescent wings with a white patch near the tip, and a metallic green body.

Comparing with similiar family species Calopteryx virgo (Banded Agrion), the difference exists in extention of the band in the wing patch.  The wing patch of C. splendens starts at the nodus but can reach up to the wing-tip in southern races. The nodus is the sharp dip mid-way down the upper edge of the wing. That of the C. virgo, the colouring on the wing starts before the nodus, further towards the body, than the Banded demoiselle.

Demoiselles are damselflies that belong to the family group called "Calopterygidae". This family includes two large damselfly species: the Banded Demoiselle Calopteryx splendens and the Beautiful Demoiselle Calopteryx virgo. Demoiselles have a graceful butterfly-like fluttering flight and are usually seen flying over riverside vegetation.

Males are usually territorial, but large numbers can sometimes be found in lush bankside plants and on floating objects. They court females by opening their wings and performing an aerial dance. They are usually found among mature, slow-flowing streams and rivers and sometimes canals.

The eggs hatch after 14 days. The larvae have very long legs and are stick-shaped. They develop over two years, usually, live amongst vegetation and tend to be most active at night. They tolerate muddy water and overwinter buried in mud. When they are ready to moult into an adult, larvae leave river and may climb up a suitable reed or plant, move up to 100m prior to emergence and shed their skin.

Adults take 7-10 days to mature after emergence and do not appear on breeding sites until then. Mature males either hold territories on suitable egg-laying sites or perch on riverside vegetation. Territorial males will court females, perching males attempt to mate with any female. Females only visit rivers when ready to oviposit.

The usually appear end of April to September according to locality.