The Gesellschaft fur Anlagen-und Reaktorsi-cherheit(GRS)gGmbH as the main technical support organization for the German Federal Government in nuclear safety has been dealing with small modular reactors(SMRs)for about one decade since SMRs are one interesting option for new builds in most countries worldwide which continue to use nuclear energy for commercial electricity production.Currently four different SMR designs are in operation,four in construction,one is licensed,and further 12 are in a licensing process.In this paper,definitions,history,and current developments of SMRs are presented.Subsequently,selected trends of SMR development such as factory fabrication and transport,compactness and modularity,core design,improved core cooling,exclusion of accidents,features for preventing and limiting the impact of severe accidents,economic viability,competitiveness and licensing are discussed.Modeling gaps of the GRS simulation chain programs with a view to applications in nuclear licensing procedures are identified and a strategy for closing these gaps is presented.Finally,selected work on the extension and improvement of the simulation chain and first generic test analyses are presented.