The formulation of API Code 27: Standard Procedure for Determining Permeability of Porous Media, provided the oil industry with concepts, definitions, and reliable means of obtaining quantitative data for this important physical property of rocks containing oil and gas. Progress was rapid thereafter, experience and data accumulated in the laboratories and files of the industry, and anomalies began to be observed and reported. The questions were asked, "Do we know how to measure permeability? If so, how can the anomalies be explained; and if not, what should we do?" This paper reports the results of a comprehensive research project at The Pennsylvania State College, sponsored by the American Petroleum Institute, undertaken and completed to answer thise questions. The permeability to air and related data for 164 cores from various oil fields of the United States, from California to Pennsylvania, are reported. The importance and magnitude of the Klinkenberg effect in the measurement of permeability was determined, and practical evaluation of the effect for routine testing is suggested. The permeability of many of these cores to liquids, including petroleum fractions and salt water, was obtained. The experimental results have been correlated with theory; and, as a result of this study, general conclusions made for the interrelation between permeability of porous media to air, water salt solutions, and non-reactive liquids,previously recognized by API Code 27 (2nd Edition) are substantiated. Consequently, this study provides data which are quantitative proof of the soundness of the theory underlying the behavior of fluids in porous media.