Since 1975, the heads of state or government of the major industrial democracies have been meeting annually to deal with the major economic and political issues facing their domestic societies and the international community as a whole. The six countries at the first summit, held at Rambouillet, France in November 1975, were France, the United States, Britain, Germany, Japan and Italy. They were joined by Canada at the San Juan Summit of 1976 in Puerto Rico, and by the European Community at the London Summit of 1977. From then on, membership in the G7 was fixed, although 15 developing countries' leaders met with the G7 leaders on the eve of the 1989 Paris Summit, and the USSR and then Russia participated in a post-summit dialogue with the G7 since 1991. Starting with the 1994 Naples Summit, the G7 met with Russia at each summit (referred to as the P8 or Political 8). The Denver Summit of the Eight was a milestone, marking full Russian participation in all but financial and certain economic discussions; and the 1998 Birmingham Summit saw full Russian participation, giving birth to the G8 (although the G7 continued to function along side the formal summits). At the Kananaskis Summit in Canada in 2002, it was announced that Russia would host the G8 Summit in 2006, thus completing its process of becoming a full member.
Link:
http://www.g7.utoronto.ca/what_is_g8.html