Ordinary clays are more or less fine grained; those containing smectite clay minerals have extraordinary fineness. Smectite clays are known to be physically unstable by sorbing or giving off water in conjunction with swelling or contraction much more than illitic or kaolinitic clays, which makes them particularly sensitive to changes in water content. In road construction, they are hence feared, but kept confined in boreholes or for isolation of hazardous waste down in the rock and in the form of landfills with sufficient overburden. They are very popular because of their tightness, but they are chemically unstable and change their behavior if the chemical conditions in the environment are altered. Smectites are favorites in cosmetics and dermatology because of their sorption potential and ability to interact well with the human skin. We will focus on these and other properties of clays that contain smectite, which is the dominant component of bentonite.