The MHD behaviour connected with edge localized modes (ELMs) is studied using fast sample rate fluctuation diagnostics. It is found that ELMs on ASDEX are generally preceded by a coherent MHD mode of medium poloidal mode number (m=10-15). At the onset of the ELM the mode behaviour changes to a turbulent one, leading to an outflux of heat and particles. These results are related to a model by Kerner and Jakoby, as described in Nucl. Fusion 29 (1989) 1959 (Section 7.2), which explains the ELM as a nonlinear coupling of free-surface modes of resistive ballooning character. This model is in good agreement with the experimental findings