Light Ampli cation by Stimulated Emission of Radiation(LASER) is a mechanismrnfor amplifying electromagnetic radiation, often visible light, via the process of stim-rnulated emission. Coherence, monochromaticity,collimation, spectral brightness andrnhigh intensity are the main characteristics of laser light.rnThis project describes the basic aspects of a laser. It includes a quantitative dis-rncussion of the major properties, the di erent types of lasers, characteristics of laserrnlight and applications of laser as well as the basic mechanisms of producing laserrnlight. Moreover, the Einstein A and B coe cients and their historical derivation arernreviewed. In addition, time-dependent perturbation theory is used as a tool in un-rnderstanding how electromagnetic radiation interacts with atoms, causing transitionsrnof its electrons among di erent energy levels