The Type I ELMy H-mode regime is the reference regime of next step devices such as ITER. Discharges that fulfil the ITER requirements in terms of energy confinement, BetaN and plasma density have been recently achieved at JET. A major drawback of the Type I ELMy H-mode is the periodic large energy losses associated with the Type I ELMs, which can lead to intolerable erosion rates when extrapolated to ITER. Hence, it is necessary to understand the physical mechanisms that determine the Type I ELM size in H-modes to assess whether the ELMy H-mode is a fully extrapolable regime to next step devices.