Almost all experiments in the high energy physics involve collisions of particles. The description of such processes in quantum field theory models requires the identification of scattering states i.e. states which in the asymptotic past or future correspond to a specific configurations of particles. During the talk I will present the construction of scattering states of massless particles and prove that the Hilbert space formed by these states has Fock structure. Despite the long range correlation inevitable in the presence of massless particles the construction turns out to be simpler than the analogous construction for massive particles.