The aetiology of copper deficiency in grazing ruminants has been clarified by a number of recent discoveries: the low availability of copper in lush grazed pasture compared with conserved forage; the inhibitory effects on absorption of small increases in herbage molybdenum and sulphur and the antagonism from iron ingested in soil; and the wide genetic variation in copper absorption between different breeds of sheep. The economic importance of copper deficiency has been emphasised by the discovery of unsuspected causes of loss: increased susceptibility to infection and growth retardation in lambs and infertility in cattle. The diagnosis of functional copper deficiency has been improved by the addition of erythrocyte superoxide dismutase to the assays of copper status.