The evolution of planetary nebulae is controlled largely by hardening of the radiation field from the central star and by hydrodynamic interactions between the “fast wind” and the slower red giant wind. These processes also result in the heating and dissociation of H2 and in the production of H2 vibration–rotation lines in the near-infrared. Both mechanisms tend to produce high gas temperatures and, at high densities, a thermal population of states. Kinematic studies provide vital information on the geometry and expansion of the nebulae and offer a discriminant between shocked and photodissociated regions.