The phenomenology of edge localized modes (ELMs), an MHD instability occurring in the edge of H-mode plasmas in toroidal magnetic fusion experiments, is described. ELMs are important to obtain experimental control of the particle inventory of fusion plasmas. From an analysis of the ELM behaviour of different magnetic fusion experiments, three distinct types are identified, namely dithering cycles, type III and type I ELMs. A physical picture of these phenomena is established on the grounds of theoretical models put forward to describe the different ELM phenomena.