Measurements of a number of thermal parameters [e.g., temperature (T), the geothermal gradient (G or Γ), heat flow (Q or q), heat generation (A or H), heat conductivity (λ or κ), heat capacity (c) and heat diffusivity (a)], interpretation of thermal measurements and analysis of thermal regime of different layers of Earth (e.g., crust and lithosphere) are among the main tasks of geothermics. Since temperature (T) is one of the key parameters used in thermodynamics, correct determination of temperature at any depth within the Earth where it cannot be measured is an extremely important problem. Thermodynamic regime is also important for analysis of conditions for generation and preservation of oil and gas fields, as well as such events as metamorphism and volcanism as well as many other processes. The development of a geothermal model of the medium, methods of geothermal regime analysis, problems of heat absorption in the Earth's strata and theory of heat absorption are discussed. It is shown that advanced methods developed in magnetic prospecting may be applied for quantitative interpretation of thermal anomalies. The Chapter is finalized by considering the models of strongly nonlinear thermal phenomena and thermal precursors of earthquakes.