This is correct. You may only use Half-Life. ESF can not run on any side mods like Source. Downloading the image and getting a key doens't work either. I did that and all it did was give me trouble. I went out and bought the last copy of Half-Life were i live. Was about 17 bucks but it came with valves 4 other games. Once you have Half-Life, install it, then go to your steam account and click on browse games. In the bottom left corner you will see a button that asked you to register your games. Put in the cd key that came with the game. Once its done, download esf and start to play. Trust me, it make seem like a hastle to do but it's completely worth it.Saiyan_Overlord said:You need half-life 1 to play esf. It's only about 10 or 20 bux for it anyway. No part of hl2 or the hl2 deluxe edition with hl redone on source can not be used for esf.