Members of ICMI are neither individuals nor organisations, but countries. There are two categories of membership — namely those countries that are members of IMU and other countries specifically co-opted to the Commission — each elaborated below.
The member states of IMU automatically become members of ICMI. The so-called Adhering Organization (AO), typically the academy of science, the mathematical society or suchlike, monitors the membership of IMU for a given country. The AO appoints in turn 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 as well, or of the Sub-Commission for ICMI (see below), when it exists.
The second category of ICMI members consists of the countries that are non-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 of coordinated involvement of the major parties in mathematics education within the country at issue, such as organizations of university and research mathematicians, researchers in mathematics education, teachers of mathematics, etc., 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 90 member countries of ICMI, 69 of which are also members of IMU and 8 are associate members of IMU. Each state, whether an IMU country 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. Moreover every four years, the Representatives elect the ICMI Executive Committee during the ICMI General Assembly.
In quite a few countries, 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 exchange of information 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.