This chapter discusses various aspects of the endocrine system in detail. Isolated endocrine cells are distributed in the epithelial lining of an organ. Recent studies have shown that cells in many other locations in the body produce amines that have endocrine functions. The pars anterior and the pars intermedia, collectively referred to as the adenohypophysis, are made up of cells having a direct secretory function. Both acidophils and basophils can be divided into sub-types on the basis of the size and shape of the granules in them. Sections of the pineal gland stained with haematoxylin and eosin has presence of irregular masses made up mainly of calcium salts. The thyroid gland contains C-cells which intervene between the follicular cells and the basement membrane. C-cells secrete the hormone thyro-calcitonin that has an action opposite to that of the parathyroid hormone on calcium metabolism. The suprarenal cortex is made up of cells arranged in cords and the innermost layer of the cortex is called the zona reticularis.