Variation in litter size of sheep can be determined by variation in both ovulation rate and embryo survival. This paper reviews the literature available on the extent of such variation and its causes. Embryo survival is inversely related to ovulation rate and shows genetic variation, but little definitive work has been undertaken to explain its physiological basis. Considerable research has been undertaken in the quest for hormonal correlates of ovulation rate by comparing patterns of hormone secretion in prolific and non-prolific breeds of ewe, but has yielded equivocal results. Manipulation of ovulation rate by using techniques such as immunization against ovarian steroids clearly suggest an involvement of the endocrine system in the determination of ovulation rate. The inability to show consistent relationships between quantitative measures of hormone secretion and ovulation rate may be attributable to the sensitivity of the techniques used to measure gonadotrophin status. In addition it may be confounded by variation in the sensitivity of ovarian follicles to gonadotrophin stimulation, a largely unexplored area.