Active control of Edge Localized Modes (ELMs) by externally applied fields offers an attractive method for next-generation tokamaks, e.g. ITER. The previous experiments on JET and DIII-D have shown that type-I ELMs can be controlled by applying static external magnetic perturbation fields. On JET, when a low n (1, 2) field with amplitude of a few Gauss at the plasma edge ( yN > 0.95) is applied during the stationary phase of a type-I ELMy H-mode plasma, the ELM frequency rises by a factor of ~ 4-5 and follows the applied perturbation field strength. This allows for ELM control in a wide range of plasma parameters. The frequency of the mitigated ELMs is propotional to the input heating power similarly to Type-I ELM, but the controlled ELMs have a higher frequency and are smaller in amplitude. In this paper, an overview of the key physics issues related to ELM control with magnetic perturbation fields is given from the experimentalist's point of view including edge magnetic ergodisation, density pump-out effect, non-resonant magnetic plasma rotation braking and multi-resonance effect in ELM frequency.