Ancient and modern types of sedimentary placer deposits formed in both alluvial and coastal environments have been signficant sources of the rare earth elements (REEs). The REE-bearing minerals in placer-type deposits are primarily monazite [(Ce,La,Nd,Th)PO4] and sometimes xenotime (YPO4), which are high-density (heavy) minerals that accumulate with the suite of heavy minerals. Monazite has been extracted from many heavy mineral placers as a coproduct of the economic recovery of associated industrial minerals, such as titanium oxide minerals (ilmenite, rutile), zircon, sillimanite, garnet, staurolite, and others. Xenotime has been produced from some alluvial deposits as a coproduct of tin (cassiterite) placer mining. Placers are mineral deposits formed by the mechanical concentration of minerals from weathered debris. Placers can be classified as eluvial, alluvial, eolian, beach, and fossil (paleo) deposit types. Monazite-bearing placer-type deposits can occur in residual weathering zones, beaches, rivers and streams, dunes, and offshore areas. The detrital mixture of sand, silt, clays, and heavy (dense) minerals deposited in placers are derived primarily from the erosion of crystalline rocks, mainly igneous rocks and moderate- to high-grade metamorphic rocks (amphibolite facies and higher). In fluvial settings, slope is an important factor for the concentration of heavy minerals from detritus. In coastal settings, the actions of waves, currents, tides, and wind are forces that concentrate and sort mineral particles based on size and density. Placer deposits containing monazite are known on all continents. In the past, by-product monazite has been recovered from placers in Australia, Brazil, India, Malaysia, Thailand, China, New Zealand, Sri Lanka, Indonesia, Zaire, Korea, and the United States. More recently, monazite has been recovered from coastal and alluvial placers in India, Malaysia, Sri Lanka, Thailand, and Brazil. In particular, along the southwestern and southeastern coasts of India, beach deposits rich in heavy minerals have experienced renewed exploration and development, partly to recover monazite for its REEs as well as its Th, to be used as a nuclear fuel source. Exploration designed to locate heavy mineral placers in coastal environments should identify bedrock terranes containing abundant high-grade metamorphic rocks or igneous rocks and identify ancient or modern coastal plains sourced by streams and rivers that drain these terranes. Trace elements associated with heavy mineral placers, useful as pathfinder elements, primarily include Ti, Hf, the REEs, Th, and U. Radiometric methods of geophysical exploration are useful in discovering and delineating deposits of heavy mineral sands. Several minerals in these deposits can produce a radiometric anomaly, but especially monazite, due to its high thorium content. Some beach districts in India and Brazil have been demonstrated as areas of high background radiation with potential dose exposure to humans and others, primarily due to the Th and U in detrital grains of monazite and zircon. Monazite- or xenotime-bearing placers offer several advantages as sources of REEs. Ancient and modern deposits of heavy mineral sands that formed in coastal settings can be voluminous with individual deposits as much as about 1 km wide and more than 5 km long. Grains of monazite or xenotime in placer deposits are mingled with other heavy minerals of industrial value. Monazite and xenotime are durable and often the heaviest minerals within the sand-silt deposit, which makes them relatively easy to mechanically separate. Thus, the REE ore minerals, monazite or xenotime, can be recovered from heavy mineral placers as a low-cost coproduct along with the economic production of the associated industrial minerals. This special volume provides a comprehensive review of the current state of knowledge for rare earth and critical elements in ore deposits. The first six chapters are devoted to rare earth elements (REEs) because of the unprecedented interest in these elements during the past several years. The following eight chapters describe critical elements in a number of important ore deposit types. These chapters include a description of the deposit type, major deposits, critical element mineralogy and geochemistry, processes controlling ore-grade enrichment, and exploration guides. This volume represents an important contribution to our understanding of where, how, and why individual critical elements occur and should be of use to both geoscientists and public policy analysts. The term “critical minerals” was coined in a 2008 National Research Council report (National Research Council, 2008). Although the NRC report used the term “critical minerals,” its focus was primarily on individual chemical elements. The two factors used in the NRC report to rank criticality were (1) the degree to which a commodity is essential, and (2) the risk of supply disruption for the commodity. Technological advancements and changes in lifestyles have changed the criticality of elements; many that had few historic uses are now essential for our current lifestyles, green technologies, and military applications. The concept of element criticality is useful for evaluation of the fragility of commodity markets. This fragility is commonly due to a potential risk of supply disruption, which may be difficult to quantify because it can be affected by political, economic, geologic, geographic, and environmental variables. Identifying potential sources for some of the elements deemed critical can be challenging. Because many of these elements have had minor historic usage, exploration for them has been limited. Thus, as this volume highlights, the understanding of the occurrence and genesis of critical elements in various ore deposit models is much less well defined than for base and precious metals. A better understanding of the geologic and...