This chapter addresses the problem astronomers are facing when using adap- tive optics: there are not enough bright stars in the sky. In summary, the spatial resolution at the focus of astronomical telescopes is limited by the poor optical quality of Earth's atmosphere. It is due to turbulence, which is always and everywhere present, even at the quietest locations such as high altitude mountains where modern observatories are built. Adaptive optics is the most e±cient tool for sharpening images to the ultimate di®raction lim- ited resolution of the telescope. It requires relatively bright reference sources close to the programme object to measure phase disturbances on the incom- ing wavefront. Such sources are so scarce in the sky that the probability is very low that one will be close enough to the faint programme object, e.g. a quasar. The remedy to bright star scarcity is to create arti¯cial ones close to the programme objects. It is achieved by pointing a laser beam in the sky at the target. Backscattering of the beam in the upper atmosphere, above the turbulent layers, can produce the required reference source. It is the so-called laser guide star (hereinafter referred as LGS). In this chapter we will shortly introduce the formation of images through a turbulent medium and its consequences for imaging in astrophysics (Sec- tion 2).We will give the principle of adaptive optics and its limitations. It will lead us to the need for the LGS. We will review the three main backscat- tering process, addressing the laser matter interaction in the atmosphere (Section 3). Unfortunately, LGSs are not natural stars. They have physical properties which cause di±culties: they are at a ¯nite distance, and they 1 cannot be used to measure the tilt of the incoming wavefront over the full aperture of the telescope. These problems will be then discussed, as well as the way to overcome them, such as the polychromatic LGS (Section 4) and the arrays of LGSs to feed multiconjugate adaptive optics (Section 5), hereinafter referred as MCAO. The laser device requirements derived from the need to provide enough return photons, given the above mentioned prob- lems are also solved (Subsection 2.3). We will discuss the most suitable laser devices to ¯t these requirements, depending on the solutions adopted (Sec- tion 6) and we will give examples of LGS devices installed at astronomical telescopes. Finally we will conclude by highlighting the main topics of re- search and development to bring the LGS concept into full operation at the largest telescopes in the world (Section 7).