The new class of Extremely Large Telescopes (ELTs) has implemented more rigorous systems engineering processes and tools for requirements management than has been used in past observatory projects. The similarities and differences between these activities at the ESO-ELT, GMT, TMT, and NOIRLab US-ELTP projects are summarized. We show that, while the key steps of the requirements management process are common among the ELTs, each project has implemented its own variation of the processes and tools tailored to its needs.