Illinois Extended Reality testbed or ILLIXR (pronounced like elixir) is the first fully open-source Extended Reality (XR) system and testbed. The modular, extensible, and OpenXR-compatible ILLIXR runtime integrates state-of-the-art XR components into a complete XR system. The testbed is part of the broader ILLIXR consortium, an industry-supported community effort to democratize XR systems research, development, and benchmarking.