A G20 summit was held in Pittsburgh on 24-5 September 2009.  Hosted by President Obama, there was quite a turnout.  Leaders in attendance were:

States

Argentina   Cristina Fernández de Kirchner President 
Australia   Kevin Rudd     Prime Minister 
Brazil   Luiz Inácio Lula da Silva  President 
Canada   Stephen Harper    Prime Minister
China   Hu Jintao     President
France   Nicolas Sarkozy    President 
Germany   Angela Merkel    Chancellor 
India    Manmohan Singh    Prime 
Indonesia   Susilo Bambang Yudhoyono  President
Italy   Silvio Berlusconi    Prime Minister 
Japan   Yukio Hatoyama    Prime Minister
Mexico   Felipe Calderón    President 
Netherlands*  Jan Peter Balkenende   Prime Minister  
South Korea  Lee Myung-bak    President 
Russia   Dmitry Medvedev    President
South Africa  Jacob Zuma    President 
Spain*   José Luis Rodríguez Zapatero  Prime Minister  
Turkey   Recep Tayyip Erdo?an   Prime Minister
United Kingdom  Gordon Brown    Prime Minister
United States  Barack Obama    President

Saudi Arabia and South Korea are also G20 members
N.B. Countries with a * are not members of the G 20

Regional organisations

New Partnership for Africa’s Development (NEPAD) represented by Meles Zenawi, Chair of NEPAD and Prime Minister of Ethiopia
Association of Southeast Asian Nations (ASEAN) represented by Abhisit Vejjajiva, Chair of ASEAN and Prime Minister of Thailand
European Commission represented by José Manuel Barroso, President 
European Council represented by Fredrik Reinfeldt, President  and Prime Minister of Sweden

International organisations

United Nations represented by Ban Ki-moon, Secretary-General 
World Bank represented by Robert Zoellick, President
International Monetary Fund represented by Dominique Strauss-Kahn, Managing Director

G20 meetings are normally attended by the finance ministers and central bank governors and the EU by the rotating Council presidency and the European Central Bank.

The most significant decision taken at Pittsburgh was probably that the group will become the new permanent council for international economic cooperation. This means that G20 meetings will essentially replace the G8, which will continue to meet on major security issues but will have reduced influence. Thus, major developing nations, such as China, India and Brazil, will have seats at the ‘top table’. 

President Barack Obama gave a snapshot of what was achieved:
• Creating a 21st Century International Economic Architecture
• Framework for Strong, Sustainable, and Balanced Growth
• Bold and Coordinated Actions from Crisis to Recovery 
• Partnering on Food Security
• Acting on our Global Energy and Climate Change Challenges
• Support for the most vulnerable

G8 is an international forum for the governments of Canada, France, Germany, Italy, Japan, Russia, UK, and US.  The EU is also represented at the meetings by the president of the Commission and the rotating Council Presidency.
 
G8 ministers meet during the year, eg G8 foreign ministers or G8 environment ministers.

Hitherto, G8 was the most important composition but its membership is out-of-date.  The EU has four seats plus the Commission, excessive by any measure.  Canada was only included to partially balance European dominance.  It is obvious that as Russia is a member, China should be admitted.

G7 is the meeting of the finance ministers from the group of seven industrialized nations of the world: Canada, France, Germany, Italy, Japan, U K and US, ie G8 excluding Russia. 

The finance ministers meet several times a year to discuss economic policies. Their work is supported by regular, functional meetings of officials, including the G7 Finance Deputies.

G2, ie China and the United States, is an idea talked about in the US buy not supported in Beijing.

G3, ie China, the EU and the US, is also a non-starter.

The G8 could become the G5: China, EU, Japan, Russia & US, bu this is unlikely.

Opinion is growing that the G20 should not be limited to financial and monetary policy.