The importance of predicting human ovulation for either optimizing or avoiding conception has been considered from an endocrine, morphological and clinical view point. Of the biochemical markers in peripheral blood, a knowledge of the LH peak is the most clearly defined, with a two to four fold increase above baseline levels for a relatively short 24-30 hour preovulatory period. Ovulation is considered to occur 28-36 hours after the beginning of the LH rise or 8-20 hours after the LH peak. Daily assessment of the rise in preovular oestrogen reflects Graafian follicle development but the rise is less distinct and spread over 3-4 days with marked day to day fluctuations. LH induces a marked reduction in oestrogen production some 12 hours prior to ovulation and at the same time induces a two to three fold increase in progesterone production above baseline levels. While these changes in themselves are not great enough for day to day discrimination, a knowledge of their reciprocal relationship may be. The preovular rise in FSH is relatively small compared to LH and the radioimmunoassay technique has not generally been refined to be as rapid and reliable. Monitoring the day to day growth of the preovular follicle ultrasonically is both linear and potentially predictable but there is a wide range of its final diameter (17-26 mm) prior to ovulation making prediction inaccurate. With further refinements in ultrasonic resolution, detection of intrafollicular changes of the cumulus oophorus and granulosal cell layer configuration and thickness may give a closer prediction of the time of ovulation. At a clinical level a knowledge of menstrual cycle length in association with body messages which herald ovulation are useful and may forewarn that ovulation in terms of days is approaching. Such markers as preovulation pain, the detection of periovular cervical mucus and the change in physical character and position of the cervix are reliable signs of preovulation for many well motivated and informed women for either promoting or avoiding conception. A knowledge of the basal body temperature is not a prospective guide to ovulation, but once the thermal shift is established in association with loss of periovular mucus symptoms, the fertile period can be considered to have passed. Because we do not have a precise and simple marker of human ovulation, it is necessary that the most suitable marker of pre- or postovulation is chosen for the particular need in a given individual.