When an electrically charged black hole is perturbed its inner horizon becomes a singularity, often referred to as the Poisson-Israel mass inflation singularity. Ori constructed a model of this phenomenon, in which the metric can be determined explicitly in the mass inflation region. In this talk I will discuss this model, and some of its generalizations. A key ingrediant in this model is the Price's law, which characterises decay properties of waves on black holes. I will present a derivation of this law.