The Alliance of Small Island States (AOSIS) is a coalition of small island and low-lying coastal countries that share similar development challenges and concerns about the environment, especially their vulnerability to the adverse effects of global climate change. It functions primarily as an ad hoc lobby and negotiating voice for small island developing States (SIDS) within the United Nations system.

AOSIS has a membership of 43 States and observers, drawn from all oceans and regions of the world: Africa, Caribbean, Indian Ocean, Mediterranean, Pacific and South China Sea. Thirty-seven are members of the United Nations, close to 28 percent of developing countries, and 20 percent of the UN's total membership. Together, SIDS communities constitute some five percent of the global population.

Member States of AOSIS work together primarily through their New York diplomatic Missions to the United Nations. AOSIS functions on the basis of consultation and consensus. Major policy decisions are taken at ambassadorial-level plenary sessions. The Alliance does not have a formal charter. There is no regular budget, nor a secretariat. With the Permanent Representative of Saint Lucia as its current chairman, AOSIS operates, as it did under previous chairmanships, out of the chairman's Mission to the United Nations.

AOSIS's first chairman was Ambassador Robert Van Lierop of Vanuatu (1991-1994), followed by Ambassador Annette des Iles of Trinidad and Tobago (1994-1997), Ambassador Tuiloma Neroni Slade of Samoa (1997-2002), Ambassador Jagdish Koonjul of Mauritius (2002-2005), Ambassador Enele Sopoaga of Tuvalu (acting chairman 2005-2006) and the present chairman Ambassador Julian R. Hunte of Saint Lucia (elected 10 March 2006).