Nudix hydrolases are a family of proteins defined by a conserved amino-acid sequence GX(5)-EX(7)REUXEEXGU, where U is a hydrophobic residue. These enzymes are widely distributed among all classes of organisms and catalyze, with varying degrees of substrate specificity, the hydrolysis of a variety of nucleoside diphosphate derivatives: nucleoside di- and triphosphates and their oxidized forms, dinucleoside polyphosphates, nucleotide sugars, NADH, coenzyme A and the mRNA cap. Nudix proteins are postulated to control the cellular concentration of these compounds. The genome of the model plant Arabidopsis thaliana contains 29 genes coding for putative Nudix hydrolases. Recently, several Arabidopsis Nudix genes have been cloned and their products characterized. This review summarizes current knowledge on these plant enzymes and discusses their possible cellular functions.