Laboratory methods for the measurement of micronutrients are usually based upon determinations in the peripheral blood. Most are relatively insensitive, but can screen for impending acute deficiency or toxicity. The metabolic responses to disease, injury and infection may alter the laboratory findings independently of dietary supply. Methods based upon the intra-cellular biochemical function of individual micronutrients are becoming available and can detect more marginal deficiencies. The investigation of general populations for micronutrient depletions largely relies upon clinical and epidemiological evaluation of responses to controlled trials of supplementation.