Geophysical logging should be carried out wherever a borehole is drilled. There are a range of reasonably inexpensive sets of logging equipment available and if the value of the initial drilling is to realised, especially if no sampling has been carried out, geophysical logging is essential. A review of geophysical logging equipment is given in this Chapter with the theory and application of each of the available logging tools provided. Geophysical logging results are best interpreted by a comparing the results of different tools. Examples are given for many different applications.