As of 2007, there are 192 United Nations (UN) member states. Each member state is a member of the United Nations General Assembly.
According to the United Nations Charter, Chapter 2, Article 4, the admission of any state to membership in the UN "will be effected by a decision of the General Assembly upon the recommendation of the Security Council." In principle, only sovereign states can become UN members, and today all UN members are fully sovereign states. However, four of the original members (Belarus, India, the Philippines, and Ukraine) were not independent at the time of their admission. Moreover, because a state can only be admitted by the approval of the Security Council and the General Assembly, some entities which may be considered sovereign states according to the Montevideo Convention are not members due to the facts that the UN do not consider them to be sovereign states, the lack of international recognition or opposition from certain members.
International organizations, non-governmental organizations, and entities whose statehood or sovereignty are not precisely defined, can only become United Nations General Assembly observers, allowing them to speak, but not vote, in General Assembly meetings.

Current members

Former members
Czechoslovakia joined the UN as an original member on 24 October 1945. On 10 December 1992, Czechoslovakia informed the United Nations Secretary-General that it would cease to exist after 31 December 1992, and that both its successor states, the Czech Republic and Slovakia, would apply for UN membership. They were admitted on 19 January 1993.

Czechoslovakia
Both the German Democratic Republic (i.e., East Germany) and the Federal Republic of Germany (i.e., West Germany) were admitted on 18 September 1973. Germany was reunified when the German Democratic Republic acceeded to the Federal Republic of Germany on 3 October 1990.

East Germany and West Germany
Tanganyika was admitted on 14 December 1961, and Zanzibar was admitted on 16 December 1963. Tanzania was formed when the two countries merged to form the United Republic of Tanganyika and Zanzibar on 26 April 1964, which later changed its name to the United Republic of Tanzania on 1 November 1964.

Tanganyika and Zanzibar
Both Egypt and Syria joined the UN as original members on 24 October 1945. The United Arab Republic was formed by an union of the two countries following a plebiscite on 21 February 1958, and continued as a single member in the UN until 13 October 1961, when Syria, having resumed its status as an independent state, resumed its separate membership in the UN. Egypt continued as a UN member under the name United Arab Republic until 2 September 1971, when it changed its name to the Arab Republic of Egypt.

United Arab Republic
The Union of Soviet Socialist Republics (USSR) was one of the five original founders of the UN in 1945 and joined the UN as an original member on 24 October 1945. On 24 December 1991, upon the imminent dissolution of the USSR, Boris Yeltsin, the President of Russia, informed the United Nations Secretary-General that the membership of the USSR in the UN (including its permanent seat on the Security Council) was being continued by Russia with the support of the 11 member countries of the Commonwealth of Independent States.
The remaining former Soviet Republics are currently all UN members:

Belarus and Ukraine had already joined the UN as original members on 24 October 1945, represented by the Byelorussian SSR and the Ukrainian SSR respectively until their independence in 1991.
Estonia, Latvia and Lithuania were admitted on 17 September 1991.
Armenia, Azerbaijan, Kazakhstan, Kyrgyzstan, Moldova, Tajikistan, Turkmenistan, and Uzbekistan were admitted on 2 March 1992.
Georgia was admitted on 31 July 1992. USSR
Yemen (i.e., North Yemen) was admitted on 30 September 1947, first represented by the Mutawakkilite Kingdom of Yemen, then by the Yemen Arab Republic. Democratic Yemen (i.e., South Yemen) was originally admitted as "Southern Yemen" on 14 December 1967, represented by the People's Republic of South Yemen, which later changed its name to the People's Democratic Republic of Yemen. Yemen was unified when the two countries merged to form the Republic of Yemen on 22 May 1990.

North and South Yemen
Yugoslavia joined the UN as an original member on 24 October 1945, represented by the Democratic Federal Yugoslavia, which later changed its name to the Socialist Federal Republic of Yugoslavia. The Socialist Federal Republic of Yugoslavia had been dissolved by 1992, and the Federal Republic of Yugoslavia was formed on 28 April 1992 by the former Yugoslav Republics of Serbia and Montenegro. By General Assembly resolution A/RES/47/1 on 22 September 1992, the UN "considers that the Federal Republic of Yugoslavia (Serbia and Montenegro) cannot continue automatically the membership of the former Socialist Federal Republic of Yugoslavia in the United Nations, and therefore decides that the Federal Republic of Yugoslavia (Serbia and Montenegro) should apply for membership in the United Nations and that it shall not participate in the work of the General Assembly."
The former Yugoslav Republics are currently all UN members:

Bosnia and Herzegovina, Croatia and Slovenia were admitted on 22 May 1992.
Macedonia was admitted on 8 April 1993, provisionally under the name "The former Yugoslav Republic of Macedonia", pending settlement over the difference that had arisen over its name.
The Federal Republic of Yugoslavia was admitted on 1 November 2000, replacing, instead of succeeding, the membership of Yugoslavia in the UN held by the former Socialist Federal Republic of Yugoslavia, which officially remained a UN member until that day. The Federal Republic of Yugoslavia changed its name to Serbia and Montenegro on 4 February 2003. Since the declaration of independence by Montenegro on 3 June 2006, the membership of Serbia and Montenegro in the UN has been continued by Serbia on the basis of Article 60 of the Constitutional Charter of Serbia and Montenegro.
Montenegro was admitted on 28 June 2006. Yugoslavia

Main article: China and the United Nations Seat of China
Since 1991, the Republic of China has repeatedly petitioned to rejoin the UN, as the representative of the people of Taiwan only, instead of all of China, using the designation "Republic of China on Taiwan", "Republic of China (Taiwan)", or just "Taiwan" (as proposed by the current Democratic Progressive Party government). However, all attempts have been denied, either because the petition failed to get sufficient votes to get on the formal agenda, or because the application was rejected by the UN, due primarily to the opposition of the People's Republic of China. The ROC has also been denied participation in the UN as an "observer state", a position held by the Holy See, or as a "non-member entity", a title currently granted to Palestine (see Observers and non-members). While the PRC government firmly opposes the application of any Taiwan authorities to join the UN or any of its agencies, such as the World Health Organization, the ROC government continues to call on the international community to recognize the right of the island's 23 million citizens to participate in the meetings and activities of the UN and its affiliates. Currently, 23 UN member states and the Holy See maintain diplomatic relations with the ROC.

Bids for membership by Taiwan

Main article: United Nations General Assembly observers