Problems relating to the anticipation and elimination of poor production due to mineral deficiencies in grazing livestock will be examined in this paper. Its concern is with subclinical problems rather than catastrophic clinical problems. The ways in which hypothetical models of animal requirements and real data on the responses of lambs to mineral supplements can be used to predict risks of deficiency will be demonstrated. In both cases mineral availability (the potential of the feed as a source of absorbable mineral) will be seen to playa key role.