Riboflavin is a water soluble vitamin that serves as a precursor of flavin mononucleotide and flavin adenine dinucleotide. These two compounds are coenzymes in a variety of electron transfer reactions that occur in energy producing, biosynthetic, detoxifying and electron scavenging pathways. When an organism is confronted with inadequate dietary riboflavin, characteristic changes occur in the cellular distribution of the various flavin fractions as well as in the activities of flavin-dependent enzymes. These changes suggest a specific hierarchic response to riboflavin deficiency, e.g. the core electron transfer chain required for ATP synthesis is preserved while the enzymes required for the first step of fatty acid beta-oxidation are diminished. The mechanisms by which the specific changes in enzyme activity are mediated have not been completely identified, but appear to result from a combination of diminished access of normal or near normal levels of apoenzyme to coenzyme and diminished abundance of apoenzyme. The changes in apoenzyme content potentially result from alterations in either protein stability or gene expression. The response to riboflavin deficiency of several key enzyme systems and the pathways affected will be discussed and a hierarchic order by which specific enzyme activities are preserved while others are decreased will be proposed. The current understanding of the molecular mechanisms by which these changes are mediated will be discussed.