In previous analysis of edge localized modes (ELMs), three different types (types I, II and III) of ELM were observed in DIII-D, whereas only one type of ELM has been found in ASDEX. The type III ELMs observed on DIII-D are similar to the ELMs observed on ASDEX, whereas the type I ELMs did not occur on ASDEX. ELMs are found to be MHD events of short duration (1 to 2 ms), which, on DIII-D, may be followed by a transient L mode phase. This compound event was not observed on ASDEX. A stability analysis of DIII-D discharges indicates that in the standard DIII-D configuration, type III ELMs occur well below the ideal ballooning limit, whereas Type I ELMs generally occur close to this limit. In DIII-D discharges where the plasma parameters are close to the ASDEX values, type I ELMs are not observed, even at the beta limit. This suggests that the ELM behaviour in DIII-D is not uniquely governed by ballooning stability