Scott Gimple will be the new showrunner on The Walking Dead beginning with Season 4, according to Hollywood Reporter.  He’ll take over from Glen Mazzara, who’s leaving after completing post-production work on Season 3 (see "Another Showrunner Leaves 'The Walking Dead'"). 

Executive producer Gimple was named Mazzara’s #2 when he was promoted to showrunner early in Season 2 and has written several epidodes. 

Mazzara’s departure has been attributed to conflict with executive producer (and comic book writer) Robert Kirkman (see "Kirkman Why Showrunner Left"), but the Reporter is now quoting "sources" as saying that Mazzara’s shortcomings became "abundantly clear... especially for the second half of the season, with production interrupted because of no material to film."