This chapter reviews the use of standard, widely available picture archiving and communication system (PACS) measurement tools to distinguish normal thymus, thymic hyperplasia, and thymic cysts from thymic tumors and lymphoma by computed tomography (CT) and magnetic resonance imaging (MRI), with the aim to reduce the unnecessary or nontherapeutic thymectomy rate and offer further guidance to thoracic surgeons when surgery is indicated. Measurement pitfalls will be discussed, along with the strengths and weaknesses of CT, 18fluorodeoxyglucose positron emission tomography (18FDG PET)/CT, and MRI in this regard. There will also be a brief discussion about the significance of tumor size, as it applies to the staging of thymic epithelial tumors. Caution is urged in ascribing too much significance to specific numbers and cutoff values provided here and in the literature in general, in the absence of other corroborative findings and the use of clinical judgment.