The electrical characteristics of the earth's crust and of the materials forming it are of great interest to those dealing with waves propagated over the earth's surface, particularly at frequencies below 106cps. In addition, recent advances in geophysical prospecting have been made by employing electromagnetic techniques. Both of these facts indicated the importance of knowing the effects of earth's crust materials upon electromagnetic waves. An over-all picture of the earth and its environment is first presented including the types of materials involved. Techniques for measuring electrical properties of materials with emphasis on in situ methods are next given. The conductivity of numerous types of earth materials is considered and results presented in tabular form for ready reference. Finally, the likely variation in conductivity with depth below the surface is considered.