The critical importance of adequate zinc status to human health, including normal growth and development, is indisputable. The high prevalence of zinc deficiency on a global basis and its importance to public health have been well documented through large-scale randomized controlled zinc supplementation trials. Similar evidence in the clinical setting, however, is much less widely available due to the nonspecific features of zinc deficiency and to the lack of sensitive biomarkers to detect zinc deficiency, especially that of a mild degree of severity. The current understanding of zinc homeostasis indicates that the primary determinants of zinc absorption are the amount of zinc ingested and dietary phytate, the latter having a major effect on zinc bioavailability. In normal as well as in many pathologic conditions, the gastrointestinal tract is the major site of zinc losses resulting from secretion of endogenous zinc into the lumen and subsequent excretion in the feces. The amount excreted is dependent on host status, the amount reabsorbed, and sometimes the presence of pathophysiologic conditions, including diarrhea and steatorrhea. Assessment in the clinical setting dictates that the clinician obtain a careful medical and diet history, recognize clinical presentations in which zinc adequacy may be compromised, and link this risk with nonspecific but plausible manifestations of deficiency. Examples discussed in this article include primary zinc deficiency due to dietary inadequacy (older breastfed infants or toddlers without zinc-rich complementary foods); genetically based deficiency (acrodermatitis enteropathica, acquired zinc deficiency of lactogenic origin), and acquired secondary deficiency in low birth weight and prematurity, gastrointestinal and hepatic disease, and cystic fibrosis. Evidence for efficacy of zinc therapy with pharmacologic doses for two conditions, Wilson’s disease and viral upper respiratory infections, is also discussed.