Members of ICMI are neither individuals nor organisations, but countries. There are two categories of membership the first being countries that are members of IMU and the second being countries specifically co-opted to the Commission.
All member countries of IMU automatically become members of ICMI. The Adhering Organization (AO) of a country (typically the academy of science, the mathematical society or similar institution) monitors the membership of IMU for a given country. The AO appoints a Committee for Mathematics to be responsible for the executive aspects of the relationship with IMU. Relations with ICMI are the responsibility of this Committee, or of the Sub-Commission for ICMI (see below), when it exists.
The second category of ICMI members consists of countries that are not IMU members. With the approval of IMU's Executive Committee, ICMI may, on an individual basis, co-opt countries that are unable to join IMU. When considering an application for membership from a non-IMU country, the ICMI Executive Committee looks for evidence within the country seeking membership of coordinated involvement from major parties in mathematics education, such as organizations of university and research mathematicians, researchers in mathematics education, teachers of mathematics, and so on according to the specific circumstances. If the application is successful, a body (normally the body responsible for the application) will be identified as an equivalent of the country's AO with respect to ICMI.
There are currently 92 member countries of ICMI, 70 of which are also members of IMU and 10 associate members of IMU. Each country, whether an IMU member or not, is invited to appoint a Representative to ICMI , who acts as a liaison between ICMI and the mathematics education community in the country. These representatives elect the ICMI Executive Committee (on a quadrennial basis) during the ICMI General Assembly.
In a number of countries the Sub-Commissions of ICMI have been established with two purposes. The first is to provide an organized local forum for dealing with issues of mathematics education and for the exchange of information related to mathematics education within the country. The second purpose is to offer an interface between the country and the international mathematics education community as represented by ICMI. The Sub-Commission includes among its members the Representative to ICMI, who is often the chairperson.