Chapter V of the United Nations Charter

Chapter V of the United Nations Charter contains provisions establishing the United Nations Security Council. 
Article 23 establishes the composition of the Security Council, with five permanent members (the Republic of China, ("currently" People's Republic of China), France, the Soviet Union, the United Kingdom, and the United States) and 10 non-permanent members elected by the General Assembly. The non-permanent members serve two year terms and cannot be immediately re-elected. In the selection process for these non-permanent members, the treaty calls for "due regard being specially paid, in the first instance to the contribution of Members of the United Nations to the maintenance of international peace and security and to the other purposes of the Organization, and also to equitable geographical distribution."
Article 24 gives the Security Council "primary responsibility for the maintenance of international peace and security" and requires the Security Council to act in accordance with the UN's purposes and principles.
Article 25 requires members to "accept and carry out the decisions of the Security Council," a mandate that does not apply with respect to the decisions of any other UN body, with the exception of the World Court's decisions once a country has accepted its jurisdiction.
Article 26 reflects a pro-disarmament sentiment:
Nonetheless, the language employed here is considerably softer than that of Article 8 of the Covenant of the League of Nations, which stated:
The Security Council made its first real effort on disarmament in June 1968 when it adopted Resolution 255, which contained assurances for non-nuclear weapon states. Some observers believe that today, the UNSC is not fulfilling its mandate under Article 26.
Article 27.
Article specifies the UNSC's voting procedures. Nine of fifteen members, including all five permanent members, must concur in order for the UNSC to act on a non-procedural measure.
The requirement of the P-5's unanimous assent establishes the famous "great power veto," which deadlocked the UNSC throughout most of the Cold War and has prevented the UNSC from authorizing military action on many occasions, including the Iraq War. It was also used by the U.S. to block re-appointment of UN Secretary-General Boutros Boutros-Ghali. There has been much debate on the veto. Some have interpreted its purpose as being primarily to prevent military action being taken against any member of the P-5; these observers therefore favor limiting the veto to that purpose and not using it for questions such as nomination of a UN Secretary-General.
Procedural measures are not subject to veto, which allows the Security Council to discuss topics over the objection of a veto holding member.
1965 amendment.
Originally, there were 11 members of the UN Security Council, with the assent of 7 (including the Permanent Five) being required for a decision. In 1965, in recognition of the growing membership of the organization, the Charter was amended to expand the UNSC to 15 members, with the assent of 9 (again including the Permanent Five) being needed to make a decision.