The Red-eyed Damselfly (Erythromma najas) is 35 mm long, predominantly black with blue markings.
Males resemble Blue-tailed Damselflies (Ischnura species) but are distinguished by their large, spaced eyes that are deep red.

The Red-eyed Damselfly is very similar to the Small Red-eyed Damselfly (Erythromma viridulum) but is bigger and less slender.
Like the Small Red-eyed Damselfly, both sexes lack pale spots behind the eyes and have pale brown pterostigmata.
Viewed from the side, the second and eighth segment of the abdomen of the male are mostly black, which distinguishes it from the Small Red-eyed Damselfly where these are mostly blue.
Immature males and females are generally yellow and black.
Another determining factor is the presence of expanses of floating vegetation on its favoured sites of still or slow moving water.

Naardermeer, Naarden, The Netherlands, 5 June 2014