In this page you can find all the current requirements in order to play Black Mesa, installation instructions and some tips to get a better gameplay experience.
Hardware + Operating System
(*) Mac isn't officially supported, but does exist a way to play BM in that platform. No port for Xbox360 (although the Xbox360 controller is supported ). Linux will be supported only if developers could port the codebase over easily .''
(**) Not including the Free Source SDK, installed separately.
Many Source games purchasable through Steam Store come with the Shared Resources, including the SDK, that you will need in order to run Black Mesa. To see if you can run Black Mesa, open Steam and look in the tools tab. If you see an entry titled "Source SDK Base 2007" installed, you can run Black Mesa.
As of July, 2011, Valve made the Source SDK free for anyone to download . This means that users who create new Steam accounts will be able to play Black Mesa without having to purchase any games.
Please note that neither the Nvidia or ATI free game packages come with the SDK base.
If the SDK Base does not appear on the tools tab in Steam you must download it in order to play the mod.
Refer the Download Page for a list of multiple mirrors and options for download. The download size is 3.8 GB. When uncompressed and installed, the game will take approximately 8 GB  of your hard-drive.
At the moment, Black Mesa cannot be downloaded/installed via Steam. The mod has been accepted on Steam Greenlight, but when the mod will be available in that platform is entirely up to Valve  (Valve Time, you know...).
Installation Instructions (step by step)
1. Install Steam http://store.steampowered.com/Steam
2. Install Source SDK 2007
- From inside the Steam interface click the dropdown menu "Library" then click "Tools".
- Scroll towards the bottom of the list, right click on "Source SDK 2007" and choose "Install game".
3. Completely shut down and exit Steam
4. Download Black Mesa
- Refer to our download page here http://www.blackmesasource.com/#download for a list of mirrors
- Go directly to Gamefront here http://www.gamefront.com/black-mesa-is-live-download-it-here/
- Scroll down to see three separate download links in green. Download all three to your desktop.
- a) blackmesa.7z
- b) BlackMesa.md5
- c) blackmesa-setup.exe
5. Install Black Mesa
- Click the file blackmesa-setup.exe to start the installation process. Be sure the game is installed to this directory: \Steam\steamapps\sourcemods\BMS
6. Launch Steam and find the game "Black Mesa" listed in your game's "Library"
Tips on hardware to get (almost) lag-free gameplay experience
You would need a videocard with a decent peak fillrate and a fast enough CPU with at least two cores. Memory requirements are at least 512MB of video card onboard memory (preferably of a GDDR5 kind; or at least of GDDRx kind and not an ordinary DDRx one) and about 1GB of free system memory available for game engine to use. Low frequency quad core CPU would perform worse than high frequency dual-core or triple core CPU.
Aim for 3GHz+ Intel Core i3/i5/i7 or 3.5GHz+ Athlon II/Phenom II or 3.8GHz+ AMD FX or AMD Socket FM1/FM2 AXX CPUs to be on a safe side. With Core i5/i7 and AMD FX CPUs SMT had been reported to cause performance regressions sometimes. You might wish to disable HyperThreding of your Intel CPU in motherboard system BIOS setup utility or set your AMD FX CPU to advertise one core per module if you happen to experience CPU-related performance issues in BM.
As for recommended GPUs: Basing on personal experiences and reports on the forums, the NVidia GTX480, GTX580 or GTX670/680 would work well. SLI configurations had also been reported to work well – i.e. it's OK to use 2xGTX460 or 2xGTX560, and dual-chip cards like GTX490/590 would also work well. Expect some micro-stutter and worse minimal FPS with SLI – this is a typical problem for this technology.