From my studies on pediatric anatomy to be issued in the near future, I am interested in beginning its publication with a paper on the macroscopic anatomy of the thymus. In this chapter I shall try to describe the thymus, its form, size, most important anatomical relations, its irrigation, and innervation. Later on I hope to explain its embryology and its histology since its physiological role is being seriously investigated and is so far the subject of conflicting opinions. I shall abstain from comments, leaving this important chapter to be dealt with by biologists, pharmacologists and physiologists.