Both functions of the mammalian ovary, the endocrine and (synthesis and secretion of steroid hormones) and exocrine (production of ova), depend upon the presence and cyclic growth of follicles, as the depletion of primordial follicles from the ovary leads to cessation of these f-unctions or female reproduction in mammals, or to postmenopausal period in humans. Actually, various fertility and sterility problems at the ovarian level are related to follicles. Therefore, a thorough understanding of the biology of ovarian follicles in mammals is of fundamental interest to a wide variety of academic and scientific disciplines. Study of their structure, function, and control involves mor­ phology, including ultrastructure, cell biology, physiology, endocrinology, biochemis­ try, immunology, neurobiology and pharmacology. Zoologists take interest in comparative and evolutionary aspects of biology of ovarian follicles in many different groups of mammals. Agricultural scientists and wildlife biologists need a thorough knowledge of the biology of follicles to control more effectively fecundity in domestic animals and endangered species of mammals. Finally, clinical scientists, toxicologists and physicians want to know the normal and pathological features of ovarian follicles in women, especially in relation to health and the regulation of fertility. Having in view the great importance of studying various aspects of biology of ovarian follicles, numerous papers and reviews on several aspects of ovarian follicles are published annually in a wide variety of scientific journals.