Eosinophils are a type of granulocyte key to immune system modulation seen in a number of disease processes. Nearly every major organ system can be connected to peripheral eosinophilia through a number of different disease processes, ranging from benign conditions to malignancy. In this paper, we review both common and rare causes of peripheral eosinophilia, their symptoms, and a framework for the workup of peripheral eosinophilia of unknown etiology.