This is actually a very good expansion for Half-Life. usually expansions stink, but this one is quite acceptable. Opposing Force takes you back to Black Mesa a one of the US Marines trying to capture Gordon Freeman (play the first half life before purchasing this). There is an extroidanary arsenal of weapons in Opp. Force and challenging new aliens that do more damage. If you liked half-life, then buy this expansion pack cause its really good. I do have one complaint: I cannot play this sucker online! i have a 500mhz Pentium 2 (its old, but still reads the data) and i have cable modem/high speed connection and it says some file differs from a server file! oh well, you can play single player over and over because its so fun and addicting. have fun playing!