We describe here 20 families of secondary (pmf-driven) carriers which, in addition to nine families within the ATP-dependent ABC superfamily, and seven families of Gram-negative bacterial outer membrane porins, largely account for the stereospecific transport of sugars and their derivatives into and out of all living cells on earth. Family characteristics as well as struc-tural and functional properties of the family constituents are described. By reference to our website (http://www-biology.ucsd.edu/ approximately msaier/transport/), phylogenetic relationships, detailed substrate specificity information and both primary and secondary references are also available. This review provides a comprehensive guide to the diversity of carriers that mediate the transport of sugar-containing molecules across cell and organellar membranes.