Chalcogenide phase-change materials are investigated for phase-change random access memories (PRAM) which is considered as the most promising next-generation non-volatile memory.The data storage is achieved by the fast reversible phase transitions between crystalline and amorphous chalcogenide phase-materials by applying electrical pulse.Understanding the mechanisms of the rapid reversible phase transitions at atomic scale is the key to designing novel phase-change materials or improving the performance of the available phase-change materials.The past years have witnessed great success in understanding this amazing phenomenon.This talk will present our progress on understanding both the crystalline and amorphous structures as well as the fast reversible phase transition mechanisms of the chalcogenide phase-change materials.