Thermal Physics deals with material properties and processes related to the conversion of heat and work and vice versa. Thermal processes can be understood at the atomic as well as the macroscopic level, and both approaches are introduced here. The laws of thermodynamics have been derived from empirical observations of gases, but they are applicable universally. The concepts of heat, work, reversibility, entropy and the steady state are central to many other areas of physics. Thermal Physics can explain the workings of heat engines, refrigerators and power stations by balancing the heat and work exchanged in each step of their operation cycle. On the atomic level, pressure can be understood as the summed impact of the collisions of gas molecules on the container walls. This is the idea of the kinetic theory, which leads to the thermal distribution of velocities and more generally to the population of energy levels. Many processes are irreversible, showing that an energy balance is not sufficient to predict the direction of a process. Entropy is introduced to take into account the dissipative nature of irreversible processes and to quantify the influence of disorder on the probability of a process.