Mobile ion contamination in CMOS devices can be catastrophic to a manufacturing facility by contaminating wafers and equipment and causing substantial financial losses. Mobile ion contamination can cause time-zero yield losses and reliability hazards for chips in the field. This chapter thoroughly covers the subject, including physics, electrical symptoms, electrical and chemical measurements, and detection. Nonvolatile memories are particularly sensitive to mobile ion contamination and are reviewed in detail. Classic and modern mitigation techniques are covered. Mobile ion problems due to alkali metals, hydrogen, and copper metallization are discussed.