A comparative study of the oil shales in the US will have very little value unless, from the analyses or assays, definite conclusions can be reached as to the quantity and quality of the petroleum substitutes and by-products that may be produced from each shale. The operator of a prospective oil shale plant desires information that will enable him to predict the quantity and quality of oil and other substances that a commercial plant will produce. These data can only be secured through very comprehensive laboratory tests. This chapter brings out those principles of testing which must be recognized and applied in order that reliable data may be obtained and logical conclusions reached.