Do I really need Steam?
Yes! Don't ask for help if you do not have Steam with Half-Life (1) registered to your Steam account!
Can you play ESF on the Source engine?
No.
Will ESF ever move to the Source engine?
No.
What is the Source engine?
The Source engine is the engine Half-Life 2 runs on. The same engine that poweres Counter-Strike : Source, Half-Life : Source, and Day-of-Defeat : Source.
What do I need to play ESF?
You need Steam, and a valid copy of Half-Life 1.
I have Half-Life version 1.1.1.0 and ESF will not work right, or at all!
That's because you aren't using Steam.
I hate Steam, I want to play without it!
You can't. ESF requires Steam to play online.
I want to play offline! Can I do that without Steam?
No.
I need a valid Half-Life 1 CD-Key. Where can I get one?
You need to buy the game at a store, or online using Steam.
I have a Condition-Zero or Counter-Strike CD-Key, will ESF work?
A Condition-Zero or Counter-Strike CD-Key will not work. You need to get a valid copy of Half-Life 1.
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There is a FAQ in the Help section for a reason.