The Hazel Dormouse or Common Dormouse (Muscardinus avellanarius) is a small mammal and the only living species in the genus Muscardinus. It is 6 to 9 centimetres (2.4 to 3.5 in) long with a tail of 5.7 to 7.5 centimetres (2.2 to 3.0 in). It weighs 17 to 20 grams (0.60 to 0.71 oz), although this increases to 30 to 40 grams (1.1 to 1.4 oz) just before hibernation. The Hazel Dormouse hibernates from October to April-May.

The Hazel Dormouse is native to northern Europe and Asia Minor. It is the only dormouse native to the British Isles, and is therefore often referred to simply as the Dormouse in British sources. [Source]