Oocyte maturation is generally defined as the reinitiation of the first meiotic division leading to metaphase II (MII), combined with the appropriate cytoplasmic processes, which are necessary for proper fertilization and early embryo development. Using in vitro maturation techniques, oocytes are harvested in the GV stage (germinal vesicle, prophase I), and matured in special maturation media. Relevant aspects of oogenesis and follicular development will be described. In vitro maturation (IVM) of oocytes was first carried out in rabbits in 1935 by Pincus and Enzmann followed by Robert Edwards, who in 1965 did the first IVM on human oocytes. Ironically, the first IVM baby was the result of in vitro maturation of an immature oocyte from a stimulated cycle (Veeck 1983). Starting with Cha in 1989 the number of babies resulting from in vitro maturation of oocytes in unstimulated cycles has steadily increased. Several reasons for using IVM can be mentioned and will be discussed in this lecture, but especially two major groups of patients are interesting for this procedure: One group consists of regularly cycling women with normal ovaries referred for IVF and ICSI due to severe male infertility problems or tubal factor. The other group consists of women suffering from PCO/PCOS. These women are extremely sensitive to stimulation with follicle stimulating hormone (FSH) and have a significant risk of developing ovarian hyperstimulation syndrome (OHSS). Collection of immature oocytes combined with in vitro maturation may eliminate the risk of OHSS. The IVM techniques are still being developed and improved, and were most recently discussed at the Second International Symposium on In Vitro Maturation of Oocytes in connection with the ESHRE Meeting in Lyon, France, on July 1 2007. At this meeting an attempt was also made to produce an educated estimate of the number of IVM deliveries and ongoing pregnancies. This estimate ended up being around 1150 until the middle of 2007. It should be mentioned that more than half of those are from Asian countries.