Two categories of membership are foreseen in ETSO: Full membership and associate membership.

Full membership is subject to the following minimum requirements:

• To be a legal person constituted under the laws of his country of origin;
• To be designated as a Transmission System Operator according to any Regulation or Directive in force concerning common rules for the Internal Electricity Market;
• the Transmission System Operator is solely responsible for the frequency control (primary and secondary) and for maintaining the energy interchange at the scheduled value within a given area (Control Area) which is located within the European Union or in a country that has entered into an agreement with the European Union providing that it shall comply with the applicable regulations regarding the Internal Electricity Market.
• To belong to a country or control area relevant to the IEM in terms of market conditions and/or the physical reality of its transmission interconnectors.

Associate membership shall fulfil at least the following minimum requirements:

• Act as a Transmission System Operator; and
• confirmation that there is a reasonable expectation that the candidate will qualify for full membership in the near future on the basis of an assessment performed by the Association and based on technical, regulatory and market conditions.