The concept of using immunological strategies to boost reproductive performance in sheep and cattle is far from new, with products such as Fecundin®, which involved active immunisation against the weak androgen, androstenedione, having been released nearly 20 years ago. However, whilst effective in sheep, immunisation against androgens was not effective in inducing multiple ovulation in cattle (unpublished observations), in which the natural rate of twinning is low (<4%). The purification of inhibin in the late 1980’s and the subsequent elucidation of its role in controlling follicle stimulating hormone (FSH) release from the pituitary represented an alternative means to increase prolificacy in domestic ruminants. Intense research effort throughout the 1990’s utilising active inhibin immunisation showed that while this approach was quite successful in sheep, results in cattle were less promising due to the extreme variability of the response obtained both between animals and within a single animals across successive cycles. Further, while numerous studies have shown that with correct identification and management production of twins in cattle can lead to significant economic gains, the possibility of more than two young clearly raised welfare concerns for both sheep and cattle in terms of dystocia and post-natal welfare of both the mother and her offspring. Over recent years, however, continued research into the control of ovarian follicle development in ruminants has suggested alternate immunological strategies that could be used to modulate prolificacy in a more reliable and controllable manner.