Paleontology has become an applied science through its usefulness in the interpretation of stratigraphy. The application is based on two laws: (1) superposition of strata, and (2) orderly chronological succession of fossils. Examples show that paleontology is applied commercially in mining and petroleum activities. A graphic analysis of the kinds of fossils which are most useful in the petroleum industry proves that the Foraminifera lead by a wide margin. Finally, the use of fossils is considered as falling into three chief categories of methods: (1) simple matching of assemblages of specimens, (2) the determination of age identity, founded on the principle that all organisms have a limited range in time, and (3) the determination of identity of depositional environment, ounded on the principle of uniformitarianism. Since the relationship between paleontology and other means of correlating is an intimate one, the applied paleontologist does not pin his faith entirely on paleontology to the exclusion of other lines of evidence, because, even employing all possible lines of attack, he is often none too certain of his correlations.