In the UK the most common tick is the sheep tick (Ixodes ricinus) and is about the size of a sesame seed (2.5 mm). It is oval, with four pairs of legs and a flattened body. It is also known as the deer tick and also the castor bean tick.

Ticks are most active in October and November and again during April and May but as the climate is becoming warmer the period of activity appears to be more prolonged. Ticks commonly attach to deer, dogs, horses and humans but are also known to infest other forms of wildlife such as woodland and migratory birds, mice, other small rodents, hares, badgers and foxes.  Ticks are known to transmit several diseases including Borreliosis (Lyme disease), Ehrlichiosis, Babesiosis, Bartonella (Cat Scratch disease) in humans and animals, Louping ill and Tick Fever in sheep.

During Autumn and Spring the adults may be found "questing" – waiting in ambush on vegetation from ground level to about 18 inches high (deer belly height) for a suitable host to pass by.

Unfed females are rusty red with a small black shield on the back, and males are smaller and uniformly dark. After feeding for up to ten days on any mammal including humans, the adult female swells to the size of a small pea, and becoming blue-black.

Most people, when asked to describe a tick, refer to a coffee bean sized insect which is like a grey-blue balloon. This description is how most people observe ticks on dogs or hedgehogs. This is the form of an engorged adult, which can be quite clearly seen however a fully fed nymph tick is much smaller and less noticeable.