During the past few years, substantial progress has been made to understand the enzymology and regulation of nucleotide sugar interconversion reactions that are irreversible in vivo on thermodynamic grounds. Feedback inhibition of enzymes by metabolic end products appears to be a common theme but some experimental results on recombinant enzymes are difficult to interpret. Using a combination of metabolic flux analysis, enzyme assays, and bioinformatics approaches, the significance of several proposed alternate pathways has been clarified. Expression of plant nucleotide sugar interconversion enzymes in yeast has become a promising approach to understand metabolic regulation and produce valuable compounds. In a major advance for the understanding of the synthesis of arabinosylated cell wall polysaccharides, reversibly glycosylated proteins turned out to act as mutases that interconvert the pyranose and furanose forms of UDP-L-arabinose.