While the first well logs recorded seventy years ago had no provision for data quality control, the development of increasingly sophisticated logging techniques has led to the introduction of a large number of tests to validate acquired data. Log quality can be assured by stringent control of depth, calibrations, signal processing and operating procedures. This work gives a thorough description of these features. The meaningful interpretation of well logs depends on valid input. An understanding of log acquisition, and the performance of rigorous quality checks are the prerequisites for an accurate evaluation of a formation. These elements also enable log users to make decisions based on calculated risks. The book is primarily written for earth science specialists who use log data. It also addresses the needs of logging engineers who seek a better understanding of the log acquisition process. Exercises and their solutions are scattered in the book to complement practical chapters.