1. Membership in the UN Two categories: Original members (article 3);
Admitted members (article 4).

2. Admission to the UN
Article 4 Membership in the United Nations is open to all other 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 admission of any such State to membership in the United Nations will be effected by a decision of the General Assembly upon the recommendation of the Security Council.

3. Admission to the UN Requirements for admission: to be a State;
to be peace-loving;
to accept obligations arising from the Charter;
to be able and willing to carry out those obligations.

4. Admission to the UN
General requirement: only States can become members. International legal personality (effectiveness and independence); recognition is not a requirement in itself.
Special cases:
micro-States;
neutral States.

5. Admission to the UN Conditional admission. Re-admission.
Representatives of States in the UN: permanent delegations.
Credentials issued by the State and submitted to the UN. Formal and substantive examination.
Antagonistic delegations.
Refusal to accept credentials as a sanction.

6. Suspension of membership
Article 5 A Member of the United Nations against which preventive or enforcement action has been taken by the Security Council may be suspended from the exercise of the rights and privileges of membership by the General Assembly upon the recommendation of the Security Council. The exercise of these rights and privileges may be restored by the Security Council.

7. Suspension of membership
Article 19 A Member of the UN which is in arrears in the payment of its financial contributions to the Organization shall have no vote in the General Assembly if the amount of its arrears equals or exceeds the amount of the contributions due from it for the preceding two full years. The GA may, nevertheless, permit such a Member to vote if it is satisfied that the failure to pay is due to conditions beyond the control of the Member.

8. Expulsion from the UN
Article 6 A Member of the United Nations which has persistently violated the Principles contained in the present Charter may be expelled from the Organization by the General Assembly upon the recommendation of the Security Council.

9. Withdrawal from the UN No express provision. Practice: Indonesia.
Issues connected to State succession.

10. Permanent observers No provision in the Charter
Special status for non-Member States (Palestine), Holy See, other IGOs, Red Cross, etc.
European Union: special status of observer since 2011.
NGOs: Article 71of the Charter. Consultation by the Economic and Social Council.