While there were many skin care breakthroughs at the end of the last century, certainly one of the greatest was the discovery and application of Alpha Hydroxy Acids (AHAs). Alpha-hydroxy acids are naturally occurring acids, derived from the sugars of selected plants and fruits such as Sugar Cane, Grapes, Citrus Fruits, Apples and Bitter Almonds. These acids perform at the base of the stratum corneum, dissolving the cement that holds dead skin cells together. Because this increases cell turnover, this results in skin that is more flexible, more smooth, and more even in tone. The main difference between alpha hydroxy acids and beta hydroxy (salicylic) acid is solubility. Alpha hydroxy acids are water soluble only, while beta hydroxy acid is lipid soluble. This means that beta hydroxy acid is able to penetrate into the sebum-filled pore and exfoliate the dead skin cells that have built up and clogged it. Because of this beta hydroxy acid is better used on oily skin with acne. Alpha hydroxy acids are better used on thickened, sun-damaged skin.