While Episode 1 is already well underway a chat with Valve head mad Gabe Newell by the magazine revealed that Episode 2 is already underway by a separate development team at Valve. Furthermore, once Episode 1 is completed later this year, that dev team will begin work on Episode 3. Episodes 1-3 will basically be a trilogy that will conclude the storyline that started in the original game. And yes there is even an Episode 4 in the works but that section is actually a stand alone story being handled by an unnamed team outside of Valve. The article also talks about Valve perhaps taking the Half-Life universe into other game genres, with Newell saying his dream idea is a Half-Life based third person tactical game.