"Multilateral negotiations regarding Iran's nuclear program date back to 2003 after the International Atomic Energy Agency (IAEA) reported on the existence of clandestine nuclear facilities at Natanz. In October of that year, Iran concluded an agreement with France, Germany, and the United Kingdom under which Iran temporarily suspended aspects of its nuclear program, including enrichment of uranium, and signed an Additional Protocol to its IAEA safeguards agreement, but also asserted its right to develop nuclear technology. In January 2006, Tehran announced that it would resume research and development on its centrifuges at Natanz. After that time, Iran held multiple rounds of talks with China, France, Germany, Russia, the United Kingdom, and the United States (collectively known as the P5 + 1)."
CRS Report for Congress, R43333
Congressional Research Service: https://crsreports.congress.gov/