This is the three-hundred and second anniversary of the first description of Harderian glands by Johann Jakob Harder. Despite more than three centuries of study, many features of these glands, including their definition and function, are not yet established to everyone's satisfaction. Some topics of Harderian biology will be reviewed. Morphology, ontogeny: Mammalian Harderian glands secrete largely by exocytosis, which is unusual in a lipid-secreting exocrine gland. Polytubular complexes are features of male hamster glands; their significance is unknown. Porphyrins: High levels of porphyrin in rodent Harderian glands have made this an attractive organ for research on the control of porphyrin synthesis. Attempts to use it as a model of human porphyria, however, have been disappointing. We do not know the function of Harderian porphyrin. The golden (hamster) age: Syrian (golden) hamster Harderian glands show marked sexual differences, including porphyrin levels. This has made them a very useful experimental animal for endocrine studies. The pineal connection: Rodent Harderian glands contain melatonin. Pinealologists often also study Harderian glands, and Harderianologists also often study the pineal. This relationship has contributed valuable insights to our knowledge of both systems. Lipids: Rodent Harderian glands are about 20% lipid by wet weight, and the main secretory product is lipid. The main lipid in many animals is 1-alkyl-2, 3-diacylglycerol. Harderian gland lipid is extensively studied for insights into lipid biochemistry; less has been done on understanding the function of lipids in the physiology of the glands. Behavior: Lipids may function as vehicles for pheromone secretion. In addition, the possible use of Harderian secretion during cold temperatures by gerbils has been noted. Immunology: Bird Harderian glands are probably lymphoid organs. This does not seem to be true of mammals, though the female Syrian hamster shows large numbers of mast cells. Harderian glands of amphibians and reptiles: Research on nonmammalian Harderian glands has been limited, though important insights continue to emerge from these studies.