When you purchase Defense Grid 2 on Steam, your key works for Windows, Mac, and SteamOS/Linux. If you wish to check out the SteamOS & Mac versions prior to October 14th, you will receive access to early-but-not-complete versions of Defense Grid 2 that are lower in performance than the final game will be, and which still have some bugs and issues that will be updated and resolved in time for an October 14th ship. We apologize for not having all 5 platforms that we targeted ready for September 23rd, but Mac and SteamOS are very close and will be final soon.