Thermoelectric materials directly convert heat gradients into electrical energy, useful for the recovery of wasted heat generated by different processes. This article describes the thermoelectric effects, the physics of their origin, and the design and efficiency of thermoelectric generators. The different transport properties of solid state materials will be illustrated with basic chemical concepts to connect the physics and chemistry in solids. State of the art thermoelectric materials will be introduced to demonstrate the typical approaches and the influence of chemical and structural influences for a maximum thermoelectric efficiency.