Energy harvesting is one of the ways explored nowadays to face the problem associated to electrical power needs. Typical powers involved are limited but the main advantage offered by the harvesting approach is the capability to achieve a localized power generation enabling a reduction in losses due to power distribution. Thermoelectric technology represents one of the solution for energy harvesting allowing the direct conversion of a collected or wasted heat into electrical power. This short lecture wants to provide the basic concepts associated to the thermoelectric harvesting. The work discusses the ideas associated to the optimization of a thermoelectric device and the design of a thermoelectric system for electrical power production. The discussion focuses the attention on the different critical aspects a researcher has to face in order to produce an operating system able to match the constrains of a target application. To conclude, a description of some reference cases is reported introducing the reader to the technological impact, actual and possible, of an old technology living a renewed interest promoted by a new environment based on new paradigms for power generation and distribution, and widespread diffusion of low-power electronic devices.