The combination of two sets of molecular tools, namely yeast expression cloning and the possibility of constructing transgenic plants, has allowed analysis of the transport processes occurring at the plasma membrane in higher plants. To date, more than 30 different plant genes for plasma membrane transporters of sugars and amino acids have been identified, mainly by expression cloning. Furthermore, the functional expression of genes in Schizosaccharomyces pombe, Saccharomyces cerevisiae and Xenopus oocytes has been applied to obtain detailed information on the biochemical properties of the transporters. The expression systems have also allowed the purification of the proteins for structural analysis and to study structure-function relationship using mutagenesis approaches. A number of mutants and transgenic plants defective in certain transport properties are available and these will help in understanding the physiology of the long-distance transport of assimilates.