This chapter examines the normative development of the veto system based on two fundamental rationales: ‘veto as a right’ and ‘veto as a responsibility.’ It argues that while Article 27 (3) of the Charter was originally introduced based on these two rationales in 1945, the un member states have made continuous efforts to produce a gradual normative shift from ‘veto as a right’ to ‘veto as a responsibility.’ This chapter consists of three main parts. First, it critically reviews the drafting history related to veto power and clarifies its two fundamental rationales. Second, it examines how the balance between the two fundamental rationales was recognized during the Cold War. In so doing, it seeks to apply these rationales to the British and French vetoes in the Suez Crisis (1956) as a classic inter-state conflict, which served as an important precedent for future veto reform. Finally, this chapter examines how the balance between ‘veto as a right’ and ‘veto as a responsibility’ has gradually shifted since the end of the Cold War. It looks at the Chinese and Russian vetoes in the Syrian case (2011~) as a salient intra-state conflict involving mass atrocities, which impelled many member states to move toward the recent veto-restraint proposals. This chapter shows that their recent inclination to adhere to the ‘veto as a responsibility’ rationale allows the Security Council to work in a timely and decisive manner to achieve the evolving un purposes and principles in the 21st century.