It is proposed that the Harderian glands are those ocular glands that have tubuloalveolar endpieces with wide lumina and secrete lipid by a merocrine mechanism. The Harderian gland occurs in various mammals, not only in eutherias but also in marsupials. However, as the gland has hereto-fore been studied almost exclusively in rodents and lagomorphs, it should be studied more in other groups of mammals, especially primitive mammals such as marsupials and insectivores. The secretory duct of the mammalian Harderian gland, as a rule, is single and opens near the third eyelid in the inner canthus. The whale Harderian gland is an exception as it is present as two masses and each of which has multiple excretory ducts. In rodents and lagomorphs, the orbital venous sinus is very well developed and surrounds almost all the orbital contents. Therefore, the surface of the gland is macroscopically smooth. Histologically, the gland has branched tubuloalveolar endpieces with wide lumina, and rarely has a duct system. It is quite difficult to detect a duct in usual histological sections. In rodents, the alveoli often contain two types of glandular cells, however, the significance of the difference between these two types is quite uncertain. The Harderian gland is the sole example in which glandular cells secrete lipid by a merocrine mechanism. The formation process of the secretory vacuoles within glandular cells is not known.