First observed on fractal drums and fractal acoustic cavities, localization of Helmholtz eigenmodes has been found to exist whatever the shape and characteristic sizes of a cavity with an irregular geometry. This is true both for shallow electromagnetic cavities and shallow acoustical cavities. More irregular structures induce a global decrease of the existence surface or participation ratios of the eigenmodes. These irregular cavities present efficient damping properties and there is a specific enhancement of the dissipation for those modes that are localized near the cavity boundaries by what we call “frontier localization”. In a different manner, in cavities partially filled with an irregular shaped absorbing material, there appears a new type of localization, “astride localization”, for modes which exist in both regions absorbing and non-absorbing regions. It is these modes that are specifically efficient in dissipating the energy of waves reaching irregular absorbing structures.