Two hundred years ago, at the time of the founding of King’s College, the idea of a geologic time scale had not yet been conceived. Fossil plants and animals, though no longer considered freaks or sports of nature, were generally regarded as remnants of organisms killed and buried by the Noachian flood. Fossil fruits and cones gave conflicting evidence as to whether the flood had occurred in the spring or the fall of the year. In the following century most of the basic principles upon which the science of Geology is founded were conceived. Fossils became recognized as the record of a succession of different faunas and floras, representing not one but many periods of time. A geological time scale was developed, its Periods based upon the successive rock Systems. Correlation by fossils extended the units beyond the limits of type areas. Gradually catastrophism as an explanation of organic changes became replaced by uniformitarianism and its evolutionary concepts. During the last hundred years fossil plants have become increasingly important and reliable in geochronology, both as complementary and supplementary to the record of fossil invertebrates and vertebrates. The extension of the geologic ranges of many plant groups has not detracted from the recognition of the successive rise and dominance of progressively more complex and more advanced floral types. Within these major floral types the use of subordinate groups in more precise dating of sedimentary units has come to depend upon many factors, including adequate sampling, well-preserved specimens, accurate identifications, proper correlations with other floras, and correct time and place relationships. Successive floras in conformable sequence have shown gradual changes in facies; floral changes have not necessarily corresponded with lithologic changes. So-called chronologic boundaries have often been difficult to relate to the rock sequence. Conflicts between the testimony of fossil plants and fossil animals have usually proven to have resulted from errors in observations, in judgment, or in interpretations, rather than from the fossils themselves. The applications of fossil plants in geochronology have varied from flora to flora. Differences in correlation methods, which reflect major differences in the nature of the plant record, are illustrated by examples chosen from (1) the Late Silurian-Devonian floras which are characterized by rapid changes in floral sequences; (2) the Carboniferous-Permian floras in which successive stages indicate widespread, stable floras; (3) the Early Cretaceous stages which show a repetition of rapid floral changes and an unusual episode of diversification; (4) the Late Cretaceous-Tertiary sequence of evolutionary stability modified by slow floral migrations; (5) the Pleistocene and post-Pleistocene succession characterized by very rapid vertical changes resulting from rapid floral migrations.