Water vapor reservoirs in young Class I systems and mature Class II disks. Region 1 lies inside the waterice line where high temperature chemistry drives all oxygen into water near the midplane (~ 10⁻⁴ abundance). Region 2 lies beyond the snow line and produces a small amount of water vapor (10⁻¹¹ up to 10⁻⁷) through photodesorption of water ice. Region 3 concerns the warm surface layers of the disk out to large radii with moderate water vapor abundances limited by UV radiation (~ 10⁻⁶). Regions 1 and 3 extend further out into the disk for the warmer Class I sources. Also, hot water can be emitted in the hot core and in outflows or shocks along the outflow cavity in Class I sources. The small cartoons at the top illustrate the larger scale disk-envelope-outflow cavity system.