Phenomenology and cosmology with superstrings

We discuss how realistic low-energy physics can arise from the ${\mathrm{E}}_8$×${\mathrm{E}}_8$ superstring. A key role is played by the discrete symmetries which typically arise after compactification. Cosmological problems are avoided because the discrete symmetry we employ is shown to be effectively embedded in a Peccei-Quinn symmetry. The models we present seem to satisfy the known phenomenological and cosmological constraints. In particular they possess a harmless axion. The existence of topologically stable superconducting vortices (strings), surviving until today, is also predicted.