European Transmission System Operators (TSOs) are entities operating independently from the other electricity market players and are responsible for the bulk transmission of electric power on the main high voltage electric networks. TSOs provide grid access to the electricity market players (i.e. generating companies, traders, suppliers, distributors and directly connected customers) according to non-discriminatory and transparent rules. In order to ensure the security of supply, they also guarantee the safe operation and maintenance of the system. In many countries, TSOs are in charge of the development of the grid infrastructure too.

40 Members from 36 countries are part of ENTSO-E.
The Observer Membership Agreement passed with TEİAŞ (Turkish TSO) was formalised on 13 December 2022.
On 22 November 2023, Moldelectrica, the Moldavian TSO, joined ENTSO-E as Observer Member.
On 1^st January 2024, Ukrenergo, the Ukrainian TSO, became Member of ENTSO-E.