Trehalose is a quantitatively important compatible solute and stress protectant in many organisms, including green algae and primitive plants. These functions have largely been replaced by sucrose in vascular plants, and trehalose metabolism has taken on new roles. Trehalose is a potential signal metabolite in plant interactions with pathogenic or symbiotic microorganisms, and herbivorous insects. It is also implicated in responses to cold and salinity, and in regulation of stomatal conductance and water use efficiency. In plants, as in other eukaryotes and many prokaryotes, trehalose is synthesised via a phosphorylated intermediate, trehalose 6-phosphate (Tre6P). A meta-analysis reveals that the levels of Tre6P change in parallel with sucrose, which is the major product of photosynthesis and the main transport sugar in plants. We propose a bi-directional network, in which Tre6P is a signal of sucrose availability, and acts to maintain sucrose concentrations within an appropriate range. Tre6P influences the relative amounts of sucrose and starch accumulated in leaves during the day, and regulates the rate of starch degradation at night to match demand for sucrose. Mutants in Tre6P metabolism have highly pleiotropic phenotypes, showing defects in embryogenesis, leaf growth, flowering, inflorescence branching and seed set. It has been proposed that Tre6P influences plant growth and development via inhibition of the SNF1-related protein kinase (SnRK1). However, current models conflict with some experimental data and do not completely explain the pleiotropic phenotypes exhibited by mutants in Tre6P metabolism. Additional explanations for the diverse effects of altering Tre6P metabolism are discussed. This article is protected by copyright. All rights reserved.