Water plays a crucial role in the operation of all polymer electrolyte membrane (PEM) fuel cells. Water is the main byproduct of power generation in the fuel cell and water is the basic carrier of protons through the membrane. Water distribution across the membrane, Naœon for our purposes in this chapter, in an operational fuel cell varies with the rate of water formation and removal, and directly determines the cell’s reliability and e¥ciency (Eikerling, 2006; Springer et al., 1991; Zawodzinski et al., 1993). Low relative humidity in the membrane will exacerbate degradation of the fuel cell (Shim et al., 2007). šus, direct monitoring of the water content distribution, spatially and temporally, in an operating fuel cell is critical to understanding PEM fuel cell behavior, and further developing fuel cell technology (US  Department of Energy, 2003). Many imaging techniques have been proposed for water content visualization in fuel cells (US Department of Energy, 2003).