The Penetrometer and Soil Exploration: Interpretation of Penetration Diagrams—Theory presents the many uses of the penetrometer for investigating soil conditions. Testing methods include the following: (1) in situ load tests on full-scale foundations; (2) laboratory testing of undisturbed samples, and (3) in situ testing of soils. The book regards the advantages of using the penetrometer as a handy tool in drilling and sampling. The text emphasizes that the investigator should never rely entirely on the analogy or the extrapolation of information pertaining to a nearby site. The text describes the different shapes of the penetrometer diagrams obtained from tests in homogeneous cohesionless soil, as well as the significance of the embedment of a pile into the bearing stratum for deep foundation designs. The paper discusses the De Beer theory, Kerisel’s theory, and the theory developed at the Delft Laboratory of Soil Mechanics. The laboratory determines the maximum soil pressure and the corresponding embedment of the pile. According to Professor L’Herminier, “the bearing capacity of a pile may be determined…from laboratory tests on soil samples, the other by extrapolating penetrometer data.” The book is suitable for structural engineers, civil engineers, geologists, architects, and students of soil mechanics.