The oral cavity can be affected by a wide variety of disorders characterized by inflammation of the gingiva and/or oral mucosa. In dogs and cats, differential diagnoses for generalized oral inflammatory disorders include plaque-reactive mucositis, chronic gingivostomatitis, eosinophilic granuloma complex, pemphigus and pemphigoid disorders, erythema multiforme, and systemic lupus erythematosus. In addition, endodontic or periodontal abscesses, infectious conditions, reactive lesions, and neoplastic conditions may initially present with localized or generalized inflammation of the oral mucosa. Determination of the underlying cause of an oral inflammatory condition relies on a thorough history, complete physical and oral examination, and incisional biopsy and histopathologic examination of lesions.