Resumen
Resolved photoionized nebulae, such as H II regions and planetary nebulae, have complex and dynamic structures, but the origin and evolution of these structures remain controversial: is it the result of inhomogeneity in the upstream gas, or intrinsic instability of the shock and ionization front surrounding the region? I describe numerical and analytic work which investigates the stability of ionization fronts, aiming to help resolve these questions.
