The upper and lower eyelids are essentially tissue flaps with conjunctiva on the inner ocular surface and haired skin on the outer surface. They contain a tarsal plate (for support), muscle, glands and cilia (eyelashes).

The outer skin is thin when compared to the rest of the body. Neither the dog nor the cat have cilia on the bottom lid. In the dog, the upper eyelid contains >2 rows of cilia, whereas the cat does not have any. Instead the first row of skin hairs are adapted for the same purpose. There are two types of gland associated with the cilia; Moll (which are modified sweat glands) and Zeis (sebaceous glands).

The other type of gland found within the eyelids is the lipid-secreting meibomian gland, which provides a component of the tear film. Closure of the eyelids is down to the action of the orbicularis oculi muscle, and the upper lid in particular has a large percentage of fast twitch fibres which provide the rapid blinking action. A complete blink is essential to distribute the tear film and maintain a healthy ocular surface. However, many breeds of both dog and cat have poor eyelid confirmation, meaning the eyelids do not slide across the ocular surface, and this can contribute to ocular surface disease.