Contrarily to the major part of the asteroids which are orbiting between Mars and Jupiter, Near Earth Asteroids have orbits which drive them close to the Earth orbit and eventually across it. Due to the potential risk of impact involved, they are intensively observed and studied, but only since the 80's. The improvement of the observing techniques and the increase of the number of ground based or space dedicated telescopes, gives us now a more realistic image of this population. In this introductory talk, we will give a general description of the context of Near-Earth Objects study, we will describe some dynamical and physical aspects, we will discuss the need of their monitoring and will shortly describe the risks that they represent.