Thelazia represent a genus of nematode parasites, which may affect the ocular and adnexal tissues of dogs and cats. This chapter describes the infection in dogs and cats with Thelazia callipaeda and Thelazia californiensis. The life cycle of these parasites comprises a definitive host (including horses, ruminants, and numerous wild carnivores) and an intermediate host. Adult parasites may be found in the conjunctival fornix, nasolacrimal system, under the third eyelid or in rare cases, inside the globe. Associated symptoms may include blepharospasm, conjunctivitis, chemosis, ocular discharge, third eyelid elevation and/or uveitis, all of which may be aggravated by self‐trauma. The diagnosis of this condition is made on the basis of clinical findings and the identification of parasites in association with ocular and/or adnexal tissues. Treatment includes the removal of adult parasites, the topical administration of demecarium bromide and/or systemic anthelmintic agents including the avermectins, praziquantel, and/or mebendazole.