One of the main functions of the skin is to maintain a competent barrier to water loss (Table 7.1). Water is continuously lost from the outermost skin layers to the atmosphere (evaporative water loss); and in order to control the rate of water loss, the barrier integrity must be preserved. Maintaining the barrier to water loss is important since hydration affects the skin’s appearance, mechanical properties, and cell signaling processes [1-9]. The barrier integrity can be compromised by chemical insult (e.g., the use of surfactant-containing cleansing products or harsh chemicals), mechanical insult, dry relative humidity conditions, and sun exposure [10-16].