The CIL Nuclear Law and Policy Team is joined in conversation by Dr Charles D Ferguson, Director of Nuclear and Radiation Studies Board at the National Academies of Sciences, Engineering and Medicine and former President of the Federation of American Scientists. This episode addresses the complex question of what is nuclear security and how it differs from nuclear safety including the difference in their accompanying cultures. The role of the design-basis threat assessment in nuclear security planning is also discussed. This is the third of a four-part series on the fundamentals of nuclear safety, security and safeguards and their interrelationships.