The science of well logging, begun by Conrad Schlumberger in 1927 as an application of his work on resistivity measurements of the earth in surface exploration, has advanced further during the intervening 33 years than even its creator could have imagined. From the early measurement of electrical properties by means of normal and lateral resistivity curves, modern logging has progressed through such changes as the introduction of wall-contacting micro-spacing devices, focused devices, dipmeters, and conductive and inductive techniques. Parameters other than electrical have been utilized, such as the radioactive properties of rocks both inherent and as a result of bombardment by both low- and high-energy neutrons; the measurement of acoustic properties the physical recovery of rock and fluid samples by wireline; etc. A chronological listing of these developmental steps of formation evaluation by downhole measurement is presented and the earlier ones are discussed in considerable detail. Each relates to a certain phase of development and is the answer to a certain need for evaluation of a specific zone or to overcome limitations of a prior method. It is hoped that this presentation of these tools, measurements, and methods, together with the interpretation techniques associated with them, will honor those who did the work and will inspire future logging engineers and analysts to even greater accomplishments than have preceded them.