Vitamin deficiencies can be detected in several ways, none of which is entirely unambiguous. Evidence of several types is, therefore, required. For instance, it is rare for clinical signs to result from a single cause, except in controlled experiments. Natural diets are rarely deficient in only a single nutrient, and individual requirements vary considerably. Biochemical and functional status indices can help bridge the gap between inadequate diet and resulting pathology. Some indices are very specific for individual vitamins; others, although only semi-specific, are useful if closely linked to tissue malfunction and hence to pathology. Ideally, biochemical indices should separate severe deficiency, mild subclinical deficiency, normal status and overload toxicity. Vitamin concentrations in plasma, serum, red cells, urine, and other accessible tissues have been used, and metabolic products of vitamin-dependent metabolic pathways have been exploited. However, many of the assays are difficult to perform and interpret, and are limited to few laboratories, world-wide. There is a need for simpler and more 'portable' tests, for routine laboratories and for the medical profession.