A primary role of β-amylase (BMY) is to produce maltose during hydrolytic starch degradation. In photosynthetic organs, BMY activity is present in the chloroplast, where substrate starch is localized. BMY activity is also found in the vacuole and cytosol, where starch is not known to be localized suggesting additional roles in glucan degradation. Transcript levels and activity of different isoforms are regulated by different stimuli including cold, heat and drought stress. However, little is known about the functional role of BMY during environmental stresses. Recently, the functionality of one of the chloroplastic forms of Arabidopsis, ct-Bmy/BMY8/BAM3, has been shown to be needed for the protection of PSII photochemical efficiency following freezing stress by presumably catalyzing the synthesis of maltose, which acts as a cryoprotectant compound and precursor of soluble sugar metabolism. Still, there remain important questions about how the mobilization of starch through maltose and glucose can lead to the accumulation of cryoprotectant soluble carbohydrates during cold acclimation.