This guide covers the measurement of thermal resistivity of soil and backfill materials to include concrete@ engineered backfills@ grout@ rock@ sand@ and any other material used to encase the cable system installed in the ground. A thorough knowledge of the thermal properties of a soil or backfill material enables the user to properly design@ thermally rate@ and load underground cables. The method is based on the theory that the rate of temperature rise of a line heat source embedded in the soil is dependent upon the thermal constants@ including the thermal resistivity@ of the medium in which it is placed. The designs for both laboratory and field thermal probes are also described in this guide. Purpose The purpose of this guide is to provide sufficient information to enable the user to select useful commercial test equipment@ or to manufacture equipment that is not readily available on the market@ and to make meaningful resistivity measurements with this equipment. Measurements may be made in the field or in the laboratory on recompacted soil samples or both.