Anisocoria is a common ophthalmic presentation in small animal practice. In the absence of other clinical signs it can be difficult to determine which pupil is abnormally sized. Our discussion will describe how to effectively approach and investigate anisocoria in the clinical patient. We will review the relevant iris anatomy as well as the neuroanatomical pathways responsible for iris dilation and constriction. We will discuss how best to evaluate these anatomical structures and how to create an appropriate diagnostic plan. Finally, we will cover several ocular and neurological causes of abnormal pupil(s) size, supported with relevant clinical cases.