Abstract Detailed comparison of published analyses leads to the following tentative conclusions about the distribution of rare metals (exclusive of the radioactive metals) in sedimentary rocks: (1) Nearly all the minor metals are somewhat more concentrated in shales than in limestones or sandstones, but the amount in ordinary shales is seldom more than twice the crustal abundance. (2) The greatest enrichment of rare metals is in the organic sediments, enrichments of more than a thousand times the crustal abundances being reported for several elements. (3) Coal is characterized by special enrichment in Ge, black shales and petroleum by V; differences with respect to other elements are probable but less certain. (4) The concentration of rare elements in phosphorite is probably due chiefly to admixed organic matter, the only elements showing slight enrichment in phosphate itself being Sr, the rare earths, and probably Pb and As. (5) Enrichment in the hydroxide sediments is relatively slight, the only exceptions being Co, Mo and Ba in manganese oxide. (6) Sedimentary silicates, carbonates and evaporites seldom show marked concentrations of the rare metals. The processes of enrichment may be divided into mechanical concentration, chemical precipitation, adsorption, and organic processes. The first is important locally in the formation of placers. The theory of precipitation processes gives information about the nature of rare metal ions in natural waters and leads to a few successful predictions about rare metal distribution. Adsorption processes and organic processes are more important than precipitation in rare metal concentration, but quantitative predictions cannot be made regarding them. Widespread enrichment of rare metals takes place in four kinds of sedimentary environment: (1) coal swamps; (2) shallow marine sediments where petroleum accumulates; (3) stagnant bottoms and open shelves in areas of upwelling water; (4) semi-arid slopes and basins where red sandstones and conglomerates are deposited. Local circumstances may lead to marked concentration in other enviroments. The facts of distribution of rare metals in rocks from the different environments make possible a few generalizations about prospecting. Kinds of research especially desirable for study of rare metals in sedimentary rocks are: (1) large-scale and systematic analytical work; (2) studies of the behavior of rare metals in experiments designed to simulate natural conditions of deposition; (3) field and laboratory work on sediments rich in one or more rare metals, with the particular aim of determining conditions of deposition as closely as possible; (4) characterization of fine-grained sedimentary rocks, especially with respect to organic matter, so that they can be readily differentiated both in the field and in the laboratory.