In vitro maturation (IVM) of human oocytes is technically mastered for a decade. The patients groups included in the studies have been PCOS, male factor infertility and other causes of infertility. Both women with normal ovaries and PCO have been included. This method was developed in which pre-ovulatory immature oocytes were aspirated without gonadotrophin stimulation or low hormonal stimulation and matured in an incubator. To increase the pool of potentially competent oocytes and to make the oocyte pick-up easier, FSH priming has been used in some selected patients, primarily in women with PCOS. The results from randomized studies have shown conflicting results, some reporting better pregnancy rates with FSH priming and some showing no beneficial effect. In vivo, gonadotropin plays an important role in the regulation of oocyte growth and maturation to trigger the meiosis resumption and nuclear maturation of oocytes in vivo. Recent studies reported that hCG priming before oocyte retrieval not only could enhance the oocyte maturation rate but also could improve the developmental potential of oocytes in vitro and increase clinical pregnancy rates. hCG priming also may aggravate nonsynchronization of oocyte nuclear and cytoplasmic maturation and which is considered to be a main reason of the poor development competence of IVM oocytes. So the effects of human chorionic gonadotropin (hCG) on oocyte maturation and development in vitro are still controversial. Of course use of hCG before immature oocyte retrieval in unstimulated ovaries make the maturation process faster and the number of MII oocytes will be higher, but there is no statistically demonstrable difference in the on-going pregnancy rate, endometrial parameters such as endometrial thickness, endometrial pattern ,or endometrial and uterine blood flow. In some centers hCG is generally added to culture medium when mammalian and human immature oocytes are cultured in vitro but recent studies indicated that the addition of hCG to in vitro culture medium did not improve the maturation rate or development potential of immature oocytes. For the IVM and development of immature oocytes from women with PCOS, hCG appears to be unnecessary. In fact in IVM method oocytes are typically retrieved from cycles which are nearly similar to natural cycles, then matured in vitro and fertilized by intracytoplasmic sperm injection. But reported implantation and pregnancy rates of this method are much lower than that achieved by IVF after controlled ovarian hyperstimulation (COH). It can be expected that up to a certain extent, the in-vitro environment could be improved in a way to better support the final maturation stages of the oocyte. Nevertheless, the stages of oocyte development before oocyte-cumulus retrieval are equally important: it is during the earliest stages of follicle development that RNA and protein stores are foreseen for later development of the oocytes until implantation. Therefore the destiny of part of the oocytes is already determined at their retrieval. It is hypothesized that cumulus cells could reflect oocyte health and bear the markers for developmental competence. Rapid measurements for these markers could be of help to the embryologist to select the developmentally competent oocytes to include in culture. Recent investigations have focused on the structural characteristics of chromatin and meiotic spindle of IVM oocytes, but little research was done concerning their chromosomal abnormalities, so their safe clinical use is still questionable. Recent Abstract of the 8th Royan International Twin Congress, Tehran, Iran, 5-7 September 2007 Yakhteh Medical Journal, Vol 9, Sup 1, Summer 2007 55 studies show the aneuploidy rate of IVM GVoocytes is comparable to the aneuploidy rate of in vivo matured oocytes and first polar bodies, regardless of the length of maturation period. Among young women affected by malignancy, the window of opportunity for fertility preservation is often limited by inadequate time to undergo ovarian stimulation or contraindication to hormonal stimulation in certain type of cancer. For these patients immature oocyte retrieval and IVM were performed prior to commencing cancer treatment. This is a new indication of IVM and embryo cryopreservation. Therefore immature oocyte retrieval followed by in vitro maturation (IVM) opens a new horizon for modern assisted reproductive technologies (ART), and recent studies in IVM make it a feasible alternative to in vitro fertilization.