Aerogels are famous for theirlow thermal conductivity making them the super-isolation materials of the future. The extreme small conductivity, which can be even close to the conductivity of vacuum isolation panels, poses, however, problems to many conventional measurement techniques, since even smallest heat leaks might give erroneous results by easily 20–30%. This chapter therefore is divided into several sections: first, general aspects of heat conduction are treated; and second, models are discussed explaining the thermal conductivity, specific heat and thermal diffusivity of porous materials, especially aerogels. The experimental techniques to measure thermal conductivity of isolating materials are discussed in detail and the theoretical background explained.