For the last several decades, molten salts have been studied for a variety of applications, including nuclear fuels and coolants for advanced nuclear reactors and electrolytes for spent nuclear fuel recycling. This talk will introduce the characteristics and uses of high temperature molten salts for nuclear energy applications. Both advantages and challenges of molten salts will be described for advanced nuclear reactors and spent nuclear fuel management. In addition, this talk will present the recent progress in electrochemical and spectroscopic methods to achieve a better understanding of how molten salts change in high temperature environments.