The aim of the course is to describe the principles of lasers and laser action. The course covers interactions of atoms and molecules with light (absorption, emission and dispersion) and includes concepts such as Doppler, collision, and natural lifetime broadening, optical saturation, population inversion, and optical pumping. The principles of laser resonators and laser action (gain, threshold, power, frequency) are discussed, and the properties of Gaussian beam propagation are introduced. The course will treat some common types of laser in more detail, including continuous-wave (cw) and pulsed, gas, dye and solid state lasers.