Until the middle of the present century, the morphology and function of the thymus were primarily of interest to those working in the fields of pathologic anatomy, endo­ crinology, and pediatrics. However, during recent decades careful and refined histo­ logic studies of the organ have expanded our knowledge. It now seems certain that the thymus plays a central role in the immune system, and some of the substances pro­ duced by this organ are considered together under the collective term of "thymic hor­ mones". In clinical medicine (in particular endocrinology and pediatrics, as well as surgery and radiologic oncology), the startling advances that have taken place in radiologic diagnostics with the advent of new imaging procedures such as computed tomography and magnetic resonance imaging have provided fresh impetus in the search for effec­ tive treatments for hyperplasia, tumors, and tumor-like changes of the thymus. Normal variants of the thymus, which lies concealed within the anterior superior mediastinum, have been recorded, and pathologic changes such as primary or secondary tumors can now be analyzed and correctly diagnosed.