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Abstract. MHD instabilities in the core and edge of the KSTAR plasmas have been visualized in 2D using an electron cyclotron emission imaging (ECEI) system with sufficient time and space resolutions for the study of the underlying dynamics. In the core region where the ECE optical depth is large, the ECEI has provided localized measurements of fast MHD phenomena such as the crash of internal kinks and coalescence of dual flux tubes. In the edge pedestal region of H-mode plasmas where the optical depth is marginal, the ECEI measurements were found to be still localized and were able to provide detailed 2D images of edge localized modes (ELMs) such as the growth of multiple filamentary structures and the crash of the pedestal characterized by fast localized bursts of the filaments [3]. The effect of electron temperature and density fluctuations on the ECE signals has been analysed to understand the limitations of the edge ECEI measurements.