This chapter describes the behavior of solid nitrogen when subjected to extreme conditions of high pressure and temperature. The evolution of molecular solids under pressure constitutes an important problem in condensed-matter physics. Nitrogen is an archetypal homonuclear diatomic molecule with a very strong triple bond. It was expected to undergo the transition of a network structure related to destabilization of its triple bond in the pressure range accessible by modern experiments. Moreover, solid nitrogen has been extensively studied theoretically, and accurate experimental data provide an important test of condensed matter theory. The chapter outlines the properties of high-pressure, low-temperature phase of nitrogen obtained by "cold" compression are different from those for the phase quenched from high temperature. Illustration is provided to show that data on the new phases are thermodynamically predicted polyatomic species, however this requires further investigation. The section on polymeric nitrogen presents the results of visible and IR absorption measurements of nitrogen at elevated pressures. References to further resources are provided.