Each Bank Group institution is owned by its member countries (its shareholders). The number of member countries varies by institution, from 185 in IBRD to 143 in ICSID, as of April 2007. The requirements for membership and the country classifications the Bank Group uses are explained in chapter 3.

In practice, member countries govern the Bank Group through the Boards of Governors and the Boards of Directors. These bodies make all major policy decisions for the organization (figure 1.1).