Pentoses represent monosaccharides with five carbon atoms. They are organized into two main groups, aldopentoses and ketopentoses. There are eight aldopentoses and four ketopentoses and each ketopentose corresponds to two aldopentoses. Only D-xylose, D-ribose, and L-arabinose are natural sugars, but others belong to rare sugars that occur in very small quantities in nature. Recently, rare pentoses attract much attention because of their great potentials for commercial applications, especially as precursors of many important medical drugs. Pentoses Izumoring strategy provides a complete enzymatic approach to link all pentoses using four types of enzymes, including ketose 3-epimerases, aldose-ketose isomerases, polyol dehydrogenases, and aldose reductases. At least 10 types of epimerases and isomerases have been used for biotransformation of all aldopentoses and ketopentoses, and these enzymes are reviewed in detail in this article.