AMC has renewed "The Walking Dead" for a fourth season, but the zombie drama will once again be making a change at showrunner, as Glen Mazzara will be leaving the series after post-production work concludes on season 3.

"The Walking Dead"  is one of the biggest hits in all of television. Among adults 18-49, it's the highest-rated non-sports show of the 2012-13 TV season, in fact. Despite that, it hasn't been immune from behind-the-scenes turmoil. Frank Darabont, who adapted the series from Robert Kirkman's comic books and ran the show in its first year-plus, was forced out midway through season 2 for reasons that still haven't entirely been explained. And now Mazzara — who was Darabont's number two and took over following Darabont's abrupt exit — is gone as well, with his exit folded into news of the show's unsurprising renewal.

"Today, AMC announces the season 4 pick-up of 'The Walking Dead,'" the network said in a statement.  "AMC also jointly announces with Glen Mazzara today that for future seasons, the two parties have mutually decided to part ways.  Glen guided the series creatively for seasons 2 and 3. AMC is grateful for his hard work.  We are both proud of our shared success.   Both parties acknowledge that there is a difference of opinion about where the show should go moving forward, and conclude that it is best to part ways.  This decision is amicable and Glen will remain on for post-production on season 3B as showrunner and executive producer." 

In a separate statement, Mazzara said, "My time as showrunner on 'The Walking Dead' has been an amazing experience, but after I finish season 3, it’s time to move on.  I have told the stories I wanted to tell and connected with our fans on a level that I never imagined. It doesn’t get much better than that. Thank you to everyone who has been a part of this journey."

And producer Gale Ann Hurd, the main constant amid these other departures, said in her own statement, “I am appreciative and grateful to Glen for his hard work on ‘The Walking Dead.’ I am supportive of AMC and Glen's decision and know that the series is in great hands with one of the most talented and dedicated casts and crews in the business. I look forward to the show's continued success.”

UPDATE: And one more statement, courtesy of Robert Kirkman (who is also a writer and producer on the series)" "I am in full support of both AMC and Glen Mazzara in the decision they have come to and believe the parties came to this decision in the best interest of the future of the show.  I thank Glen for his hard work and appreciate his many contributions to 'The Walking Dead' and look forward to working with him as we complete post production on Season 3.  I am also excited to begin work on another spectacular season of this show that I know means so much to so many people. This show has always been the result of a wide range of extremely talented men and women working tirelessly to produce their best work collectively.  I believe the future is bright for 'The Walking Dead.'  Thank you to the fans for your continued support."