This publication presents the results of the impact evaluation of one of the IAEA's flagship courses in radiation safety, the postgraduate educational course in radiation protection and the safety of radiation sources (PGEC). This course is hosted in the IAEA regional training centres. The impact evaluation covers a period of 35 years, surveying the PGEC participants from past courses taking place since 1981: this represents a rather unique case of a systematic evaluation of a training programme. The publication provides an overview of the PGEC impact on participants' professional development and career, and on the radiation safety infrastructure at organizational and national levels. Overall results of the impact evaluation are presented at regional level (Africa, Asia and the Pacific, Europe, and Latin America and the Caribbean), as well as globally.