The earth's surface and the material below it affect both the point-to-point radio-wave propagation above and below the earth's surface and the impedance of the antennas employed. For this reason the radio engineer is concerned with the macroscopic electrical properties of the earth. The properties of concern here are the conductivity and dielectric constant (or permittivity). We are also concerned with the horizontal stratification that may exist. However, we do not consider the effects of surface roughness in any detail. In what follows, the magnetic permeability of the earth is taken to be that of free space po = 4~ x lo-' H/m. Exceptions, of course, occur in localized regions containing magnetic material for which the magnetic permeability p differs from p, (for example magnetite). The techniques for determining the electrical constants of the earth can be broadly divided into two categories: (1) those where drill-hole access into the earth does not exist (2) those where drill holes can be employed Where a drill hole can be used, the methods for single drill holes give localized values of the constants in the vicinity of the hole. Propagation between two holes can be used to deduce bulk values of the electrical constants of the earth between. To characterize a broad area of the earth's surface would require data from an impractically large number of costly holes. When interest is in the earth constants over a large area, such as coverage for broadcast stations, or where drill holes do not exist, techniques are employed which may be characterized as those which use surface probes. The material in this guide is drawn from numerous articles and papers; most of it is available in the open literature. Useful summary texts are those of Keller and Frischknecht, Ref [l] and Grant and West, Ref [2]. Theoretical background is found in the prolific writings of Wait, including his book, Ref [3]. These sources treat for the most part surface-probing techniques, although one chapter of Keller and Frischknecht, Ref [l] treats some well-logging methods. Additional material is reported below with pertinent references (Ref [4]). In Section 2, we discuss techniques using surface probes and in Section 3 we discuss methods suitable for drill holes (including well logging) and, additionally, a few miscellaneous specialized cases. In what follows we exclude discussion of those arrangements which employ gamma, x-ray radiation, or similar sources of radiation. SI units are employed throughout this guide.