Accurate characterization of the primary cause and perpetuating factors is essential for successful management of ear disease in dogs and cats. Cytology is a simple, rapid, and practical diagnostic test that should be performed routinely on any and all patients presented for clinical signs consistent with otitis externa. In combination with clinical signs, otoscopic evaluation, and diagnostic testing of primary disease, serial cytology enhances the ability of veterinarians to diagnose secondary infections, monitor progression of disease, evaluate response to therapy, and make appropriate management decisions. Cytologic specimens should be evaluated for the presence, numbers, and characteristics of three key features: yeast, bacteria, and leukocytes. More than five yeast organisms or more than 25 bacteria per high-powered field is suggestive of significant microbial activity warranting therapeutic intervention. The presence of leukocytes, particularly with phagocytized bacteria, indicates "true infection" rather than overgrowth; if suppurative discharge is present, systemic therapy is needed. Cytology combined with culture and susceptibility is the best method for identification of bacterial overgrowth and infection; however, if only one test can be performed, always choose cytology. Culture results assist in the selection of appropriate antibiotic therapy, but cytology determines whether systemic antibiotics are indicated, which organisms are most significant, and when therapy can be discontinued.