There are not many common fluoride-bearing minerals that can provide a source of high concentrations of F in groundwater systems. The two most common are fluorite and fluorapatite. We demonstrate processes that affect these mineral solubilities through geochemical model simulations and then examine the evidence from groundwater data trends that support the importance of these processes. For a full appreciation of the calculations and graphic results that follow, the reader should have a basic knowledge of thermodynamics and electrolyte theory. There are many good textbooks on these subjects (e.g., Anderson, 1996; Butler, 1998; Langmuir, 1997; Nordstrom and Munoz, 1994). For the purposes of this book, this section provides a brief introduction to these topics.