Trehalose is a disaccharide that was initially thought to be rare in plants but now appears to be ubiquitous. A recent study has established that the initial step in trehalose synthesis is essential in Arabidopsis. Evidence is emerging that the precursor of trehalose (trehalose-6-phosphate) is an important regulatory molecule. In yeast, trehalose-6-phosphate regulates sugar influx into glycolysis. In plants, trehalose-6-phosphate also appears to regulate sugar metabolism, but the underlying mechanism is unresolved and may be substantially different from that in yeast.