Bacteria act to maintain their hydration when the osmotic pressure of their environment changes. When the external osmolality decreases (osmotic downshift), mechanosensitive channels are activated to release low molecular weight osmolytes (and hence water) from the cytoplasm. Upon osmotic upshift, osmoregulatory transporters are activated to import osmolytes (and hence water). Osmoregulatory channels and transporters sense and respond to osmotic stress via different mechanisms. Mechanosensitive channel MscL senses the increasing tension in the membrane and appears to gate when the lateral pressure in the acyl chain region of the lipids drops below a threshold value. Transporters OpuA, BetP and ProP are activated when increasing external osmolality causes threshold ionic concentrations in excess of about 0.05 M to be reached in the proteoliposome lumen. The threshold activation concentrations for the OpuA transporter are strongly dependent on the fraction of anionic lipids that surround the cytoplasmic face of the protein. The higher the fraction of anionic lipids, the higher the threshold ionic concentrations. A similar trend is observed for the BetP transporter. The lipid dependence of osmotic activation of OpuA and BetP suggests that osmotic signals are transmitted to the protein via interactions between charged osmosensor domains and the ionic headgroups of the lipids in the membrane. The charged, C-terminal domains of BetP and ProP are important for osmosensing. The C-terminal domain of ProP participates in homodimeric coiled-coil formation and it may interact with the membrane lipids and soluble protein ProQ. The activation of ProP by lumenal, macromolecular solutes at constant ionic strength indicates that its structure and activity may also respond to macromolecular crowding. This excluded volume effect may restrict the range over which the osmosensing domain can electrostatically interact. A simplified version of the dissociative double layer theory is used to explain the activation of the transporters by showing how changes in ion concentration could modulate interactions between charged osmosensor domains and charged lipid or protein surfaces. Importantly, the relatively high ionic concentrations at which osmosensors become activated at different surface charge densities compare well with the predicted dependence of 'critical' ion concentrations on surface charge density. The critical ion concentrations represent transitions in Maxwellian ionic distributions at which the surface potential reaches 25.7 mV for monovalent ions. The osmosensing mechanism is qualitatively described as an "ON/OFF switch" representing thermally relaxed and electrostatically locked protein conformations.