The maintenance concept provides the basis for overall dependability design requirements for Extremely Large Telescopes (ELTs), such as the European Southern Observatory's ELT, the Thirty Meter Telescope (TMT), and the Giant Magellan Telescope (GMT). The maintenance concept contains the planning of the maintenance and support policy for the complete operational system of the ELTs. The way in which the maintenance concept was developed in the ELTs is shown, and the similarities and differences in the way the operational objectives will be achieved is identified. The methodology is based on explaining seven common points (maintenance philosophy/levels; maintenance types and strategies; designing for dependability: reliability, availability, and maintainability; organizational structure and responsibilities for maintenance operations; maintenance and configuration management; key performance indicators for operations; and challenges, opportunities, and lessons learned) and condensing the knowledge during the process of generation and application of the maintenance concept.