The E-ELT has completed its design phase and is now entering construction. ESO is acting as prime contractor and
usually procures subsystems, including their design, from industry. This, in turn, leads to a large number of
requirements, whose validity, consistency and conformity with user needs requires extensive management.
Therefore E-ELT Systems Engineering has chosen to follow a systematic approach, based on a reasoned requirement
architecture that follows the product breakdown structure of the observatory. The challenge ahead is the controlled flow-down
of science user needs into engineering requirements, requirement specifications and system design documents.
This paper shows how the E-ELT project manages this. The project has adopted IBM DOORTM as a supporting
requirements management tool. This paper deals with emerging problems and pictures potential solutions. It shows
trade-offs made to reach a proper balance between the effort put in this activity and potential overheads, and the benefit
for the project.