The operational boundaries for the occurrence ofvarious types of edge localized mode (ELM) instabilityin high-confinement-mode (H-mode) fusion plasmas are reviewed.The phenomenology of ELMs and the occurrence of differentELM types in parameter space are described.Favourable ELM regimes are observed in various experimentswhich combine small ELM losses withhigh edge pressure and thereby allow for good H-mode confinementeven in situations where core confinement is strongly dependenton edge parameters.Recent progress of ELM models based on ideal or resistive MHDis described and their ability to describethe various ELM regimes discussed.