At Google I/O, Google quietly announced that “all [Chromebook] devices launched this year will be Linux-ready right out of the box.” A ZDNet article has more details.
Earlier, you could run Debian, Ubuntu and Kali Linux on Chrome OS using the open-source Crouton program in a chroot container. Or, you could run Gallium OS, a third-party, Xubuntu Chromebook-specific Linux variant. But it wasn’t easy.
Now? It’s as simple as simple can be. Just open the Chrome OS app switcher by pressing the Search/Launcher key and then type “Terminal”. This launches the Termina VM, which will start running a Debian 9.0 Stretch Linux container.