The conjunctiva is the lining tissue that covers the globe of the eye (the eyeball) and lines the eyelids and the third eyelid. Conjunctivitis refers to inflammation of this tissue.

The most common clinical signs of conjunctivitis include discharge from the eyes (watery, mucoid or mucopurulent), squinting or excessive blinking, and redness or swelling around the eyes. Conjunctivitis often involves both eyes but one eye may be affected in certain conditions. Conjunctivitis may occur with other clinical signs such as nasal discharge, sneezing or coughing.

The most common causes of conjunctivitis include bacterial and viral infections, allergies, hereditary conditions, and tumors. Conjunctivitis may occur secondary to another eye disease.

Bacterial infections that are not associated with another condition such as keratoconjunctivitis sicca ("dry eye") are rare in adult dogs.

However, puppies can have bacterial infections even before their eyes are open.