Article 4 of the United Nations Charter stipulates that membership in the UN is open to all "peace-loving states which accept the obligations contained in the present Charter and, in the judgment of the Organization, are able and willing to carry out these obligations." The original UN members were the states that participated in the San Francisco Conference, or that had previously signed the Declaration by United Nations, of 1 January 1942 and subsequently signed and ratified the Charter.

In order to become a UN member, a state submits an application to the Secretary-General, in which it formally states its acceptance of the Charter obligations. The application is forwarded to the Security Council. If the Security Council, by a vote of at least nine members (formerly seven), including all the permanent members, recommends the application, membership becomes effective on the day that it is approved by a two-thirds majority of the General Assembly. In other words, if any one of the Security Council's permanent members vetoes it, or if it fails to obtain a sufficient majority in the Security Council, the application does not reach the General Assembly at all.

Prior to 1955, there were bitter controversies and years of stalemate in the Security Council over the applications of some countries. Usually one or more of the Big Five was on bad terms with the applying state, or it would choose to withhold consent as a bargaining point against the other big powers. Finally, on December 14, 1955, by a compromise, 16 countries were admitted together. Since then, new applications, most from newly independent states, rarely caused controversy. The noticeable exceptions were the applications of the Republic of Korea (ROK) in January 1949, the Democratic People's Republic of Korea (DPRK) in February 1949; South Vietnam in December 1951; and North Vietnam in December 1951. The two Vietnams and the ROK sought action on their applications in 1975. The Security Council, by a narrow vote, decided not to take up the ROK's application, and the United States subsequently vetoed membership for the Vietnams, citing as a reason the Security Council's earlier refusal to consider the membership application of the ROK.

In response to a General Assembly recommendation, however, the Security Council in 1977 recommended the admission of the newly established Socialist Republic of Vietnam, and that country became a member in September 1977. The DPRK and the ROK maintained observer status at the General Assembly until September 1991, when both were admitted to membership simultaneously. As of October 2013, the UN had 193 member states, including 51 charter members (the 50 countries that sent representatives to the San Francisco conference, plus Poland, which ratified the charter shortly afterward) and 142 states that have joined the organization since 1945, the great majority of them former colonial territories that have achieved independence.