Plants have to adjust their water balance in response to very challenging environmental conditions such as drought, salinity, and cold but also changes in light, nutrient deficiency or soil acidity. The molecular and functional characterization of aquaporins, a class of membrane proteins that facilitate water diffusion across cell membranes, has revealed the significance of their regulation in response to these environmental stimuli. The aim of this present review is to illustrate the variety of molecular and cellular mechanisms involved. These mechanisms include the control of aquaporin gene transcription and protein abundance, stimulus-induced aquaporin subcellular relocalization, and channel gating by reversible phosphorylation or by intracellular protons. The emergence of novel mechanisms of regulation by hetero-tetramer formation or through control by reactive oxygen species, and osmotic or hydrostatic pressure gradients is also discussed. These various mechanisms do not function individually and a challenge for future research will be to understand how plants respond to stresses by integrating these mechanisms in time and space, to constantly adjust the water transport and solute transport properties of their membranes. Genetic manipulation of aquaporin functions and in particular ectopic expression of deregulated aquaporins in transgenic plants provide promising strategies to address such questions.