The development and acceptance of bioengineering techniques to quantitate skin function has revolutionized the evaluation of personal care products. Many investigations were plagued by inconsistencies related to visual assessments. The previous generation of studies were based on an observer's visual scaling and relied on remembering the previous status of the skin, so that claims of improvement or exacerbation could be made. Because evaluation scales varied between investigators, it was difficult to relate different studies to each other. Objective methods now quantify skin parameters, including transepidermal water loss (TEWL), skin surface water loss (SSWL), skin hydration, pH, color, capillary blood flow, thickness, casual sebum level, and so on and document the mechanical, physical, and functional properties of the skin. These methods are noninvasive, reproducible, and sufficiently sensitive to detect changes not apparent visually.