Publisher Summary Thermal conductivity is the capacity of a substance to conduct or transmit heat; thermal conductivities of dry rocks are the functions of density, porosity, grain size and shape, degree of cementation, and mineral composition. This chapter describes the measurement of thermal conductivity of rock/fluid systems. Two groups of methods are used to measure thermal conductivity of rocks. In steady-state methods, thermal conductivity is measured directly, while in transient methods, values of thermal diffusivity are generally measured and from these measurements, thermal conductivities are calculated. Transient methods of measurement are usually much faster than steady-state methods but results are often less accurate and it may be difficult to run such tests under simulated reservoir conditions. The comparator apparatus described in the chapter proves very satisfactory for this purpose. The apparatus needs to be carefully calibrated as the accuracy of results depends on how well this is done.