separate: one set of sister chromatids is discarded as the polar body; the other set, in response to a recrudescence of MPF, lines up on a spindle, forming the second metaphase plate (MII).Fertilization causes inactivation of MPF and ERK 2, and the second meiotic division occurs, separating the sister chromatids.One set of chromatids is discarded, as the second polar body; the other set becomes the female pronucleus.For a comprehensive description of major pathways involved in oocyte M-phase entry, see Kubiak et al., 2011 andTosti &Boni, 2011.Detailed descriptions of OMICS techniques go beyond the intent of this article; corresponding references are provided in the text.We instead focus on the most significant results obtained using these techniques, the role of large animal models in experimental designs that cannot be performed in humans, and on actual and potential contributions of different animal models to understanding of oocyte biology, with particular interest in the equine species. The mare as a model for human oocyte biologyLarge animal models allow the establishment of a wide variety of experimental designs that can not be applied in humans for obvious ethical reasons, or due to the limited and highly regulated availability of human biological samples.Among large animals, the mare has many attributes that make her a good model for reproduction in women (Carnevale, 2008).These include a long follicular phase, a long interovulatory interval (22 days), presence of a single dominant follicle, formation a large diameter follicle (~40 mm) with a large volume of follicular fluid --the same volume:body weight ratio as in women; a relatively long time from LH stimulation to ovulation (36 h for both human and horse) and, like the human oocyte, formation of a markedly dense chromatin mass within the germinal vesicle as the oocyte gains meiotic competence or undergoes atresia (Parfenov et al, 1989;Hinrichs et al., 1993).Although seasonality does not occur in women, the equine characteristic of seasonal reproductive activity provides the potential to examine the influence of applied factors when cyclic hormonal patterns are not occurring (Carnevale, 2008).Horses are the best animal model for studies on age-related infertility.Because mares can be of great value, many mares continue to be bred until they experience subfertility, thus animals with naturally-occuring age-related subfertility are available for study.Horses have a long lifespan, thus age-related subfertility occurs at an age (~20-25 years) much closer to that observed in women than is seen in other animal models.In addition, horses, unlike other large domestic species, are selected for attributes other than fertility, such as conformation, athletic prowess, or behavior.Individuals showing subfertility may be worked with intensively to try to obtain foals, thus, they provide an excellent naturally-occurring model for many intrinsic causes of subfertility.Horses have similar metabolic responses to nutrient intake to that in humans, and are used for a wide variety of athletic purposes, thus they can serve to model important physiological or pathological situations affecting reproduction in humans (such as stress, life-style, sports activity, obesity or metabolic syndrome) as well as to examine the effects of external factors such as acute or long-term exposure to drugs or environmental toxicants.In addition to mimicking the situation in humans, the development of particularly large follicles allows the possibility of collecting large amounts of mural granulosa cells (GC) issuing from the follicular wall as well as large amounts of follicular fluid (30 to 50 ml/follicle) that may be used for OMICS studies in a 1:1 comparison with the developmental status of the enclosed oocyte.The cumulus-oocyte complex (COC) www.intechopen.com