Well logging lies at the intersection of applied geophysics, petroleum and geotechnical engineering. It has its roots in the tentative electrical measurements in well bores which were made by the Schlumberger brothers some 80 years ago in the earliest days of systematic petroleum exploration. Today, a variety of specialized instruments is used to obtain measurements from the borehole during, as well as after, the drilling process. This readable and authoritative treatment of the physics of these measurements dispels the "black magic" of well log interpretation by relating them, including those obtained by the latest generation of tools, to rock physics. It offers a thorough exposé of the physical basis of borehole geophysical measurements, as well as an introduction to practical petrophysics -- extracting desired properties from well log measurements.