I wouldn't call it necessary, but HL2 and the episodes are so packed with references to HL1 that you'd be cheating yourself not to find out how it all started. And while HL1's graphics are fairly... well, simplistic compared to modern games, you can pick up Opposing Force and Blue Shift; BS comes with the high-definition models that make the visuals, if not pretty, at least more bearable. Plus Op4 was a fun expansion in itself- trudging down pitch-black sewer passages, SAW in hand, with only my night-vision goggles to show me the Voltigores lurking in the darkness... that was a memorable bit of gameplay. Blue Shift... well, it was nice, but way too short.