Hypovitaminosis A is a widespread problem, especially among preschool children in many parts of the world. According to the World Health Organization (WHO) estimates, about 100,000 children become blind every year, and many are dying due to vitamin A deficiency while millions of others are suffering from other consequences of vitamin A deficiency such as growth retardation and increased susceptibility to infection. It is, therefore, very important that not only the severe cases of hypovitaminosis A be diagnosed for immediate treatment, but also the marginal cases of vitamin A deficiency in vulnerable populations be diagnosed as early as possible so that appropriate preventive measures be implemented. Available methods for the diagnosis of vitamin A deficiency can be classified into four categories: clinical, biochemical, functional, and dietary. Clinical diagnosis is based on examining ocular and extraocular signs of hypovitaminosis A, and is only useful for the detection of severe cases. Biochemical methods are based on the plasma concentrations of retinol and retinol-binding protein (RBP), and liver reserve of vitamin A whenever possible. Two other commonly used diagnostic tests are: functional testing for nightblindness using dark adaptation time, and pathological testing for ocular signs of conjunctival xerosis, with or without the use of Rose Bengal or lissamine green dye. Dietary method for the diagnosis of vitamin A deficiency is based on the estimation of dietary intake of vitamin A and carotenoids. In addition, there are newer methods such as isotope dilution and relative dose response (RDR) techniques which have been recently proposed for the diagnosis of vitamin A deficiency. RDR appears to be a reliable and sensitive indicator of marginal vitamin A. However, the most desirable approach to diagnosis may be one based on multiple indicators for the accurate assessment of vitamin A status in the community. The purpose of this paper is to draw attention to the problems of diagnosis, to recent advances and to our Brazilian experience in this important area of vitamin A research with significant global implications.