Amid expectations for nuclear power as a carbon-free energy source are growing to prepare for climate change, deploying efforts for small modular reactors (SMRs) are being actively carried out for the application area to which existing large nuclear power plants are not proper to contribute. Sodium-cooled SMRs are also being developed as one of the candidates to lead the SMR market in the future. This manuscript introduces the background and history of the development of sodium-cooled fast reactors, and the current status of development of Sodium-cooled SMRs and microreactors that are being developed based on them.