Horner's syndrome (HS) occurs when there is interruption of the oculosympathetic pathway (OSP). This article reviews the anatomy of the OSP and clinical findings associated with lesions located at various positions along this pathway. The imaging findings of lesions associated with HS at various levels of the OSP, classified as preganglionic HS (first- and second-order neuron HS) or postganglionic HS (third-order neuron HS), are demonstrated.