The main scope of this thesis is about lasers and their application. First chapter deals with the explanation of the most important concepts that are significant for understanding the principles of how laser works. It is valuable to understand what light and photons are and how a laser beam even develops. With understanding what spontaneous and stimulated emissions are, and what an inversion of population is, the requirements for laser emission are and how it works can be set. Furthermore, the thesis deals with the structure of the laser in order to understand how its parts affect the creation of the laser beam and thus its maintenance, which is then applied in laser processes. Laser processing technologies have a variety of usages and are part of modern technologies which help in today’s microchip, solar panel, or similar industries. It’s important to emphasize the main technologies which are used today, as well as new ones which are developed and patented by individuals. In the last part, this paper deals with one of the most important tests for laser calibration – the Focus-Test. After the experiment is conducted, it is explained why correct and precise work of a laser is important and its further application.