Publisher Summary This chapter discusses the geochemistry and mineralogy of the rare earths. Rare earth abundances in natural materials have become an important geochemical tool. The rare earth elements comprise a uniquely coherent group; wherever one rare earth appears, all others are present as well. The group is coherent because under most natural conditions all members share a common (3+) oxidation state, with anomalous behavior occurring under some conditions for Ce (4+) and Eu (2+). Natural materials differ substantially from each other in concentrations of the rare earths as a group and in abundances of individual rare earths relative to each other. From the geochemical point of view, the rare earths are “dispersed” elements, i.e., spread among many common materials rather than concentrated into a select few. They are “lithophile,” i.e., when allowed to distribute themselves among common silicate, metal, and sulfide phases, they overwhelmingly enter the silicates. Geochemically, the term “rare earth” is best restricted to mean lanthanides plus yttrium. The chapter examines rare earth abundances in the solar system, and explains how rare earth abundances are used in studying rock-forming processes.