A limiting factor to realising the full potential of many of the new reproductive techniques is the lack of availability of fertile oocytes. Methods for maturing oocytes in vitro (IVM) have been developed to address this problem but the success rate and quality of embryos produced by IVM is variable. The variation in success may be due to the poor quality of oocytes that are being selected for maturation, since these would be taken from developed antral follicles. To attempt to eliminate this variation and increase the numbers produced, it may be better to use the large source of oocytes from preantral and primordial follicles by developing systems for in vitro growth (IVG). In vitro systems that utilise early growing follicles as a source of oocytes have been developed for laboratory species and these have been successful in producing live young. If successful, IVG in association with IVM would supercede existing technology for assisted reproduction in both humans and animals by making it possible to develop the desired number of high quality oocytes from small amounts of ovarian tissue. However, developing IVG systems for species with follicles that develop over several months presents enormous technical challenges. We have developed systems that permit the growth of individual porcine and bovine preantral follicles for periods of up to 20 days. Porcine follicles grown in micro-wells show a higher rate of survival if grown in the presence of serum than follicles grown under serum free conditions. Oocytes recovered from in vitro grown porcine follicles are capable of reaching metaphase II after in vitro maturation. A similar system has been developed for bovine follicles and survival rate is high under serum free conditions but as yet no oocytes from in vitro grown oocytes have been capable of completing meiotic maturation.