Nina Tassler, who has run CBS Entertainment since 2004, is stepping down as chairman at the end of the year, CBS announced on Wednesday.

CBS' current programming head Glenn Geller will succeed Tassler as the president of CBS Entertainment. Geller will begin his duties overseeing entertainment programming immediately and work closely with Tassler during the transition. Once Tassler steps down as chairman, she will remain in an advisory role through at least 2017.

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"Nina and I have been colleagues and friends for 25 years," CBS President and CEO Leslie Moonves said in a statement. "It is one of the proudest partnerships of my professional life. We've enjoyed amazing success together, both here and at Warner Bros., and it's been a privilege to watch her talents and accomplishments grow. Nina is a creative executive with the instincts of an artist, so it came as no surprise that she would eventually want to express those talents beyond television. All of us here are grateful and proud of CBS's success with Nina leading our entertainment programming, and we will be the biggest fans for all her future endeavors."

During her tenure, Tassler oversaw the successes of the The Big Bang Theory, The Good Wife ,How I Met Your Mother, Two and a Half Men and CBS' stable of procedural hits (Criminal Minds, and the NCIS and CSI franchises).

"CBS is a special place," Tassler said in a statement. "For the past 18 years, I've been blessed to be part of an amazing team that has lifted this Network to an enviable run of success. It all starts at the top with Leslie, my boss, mentor and friend for two and a half decades, and a leader who inspires excellence in every part of the creative and business process. I owe a tremendous debt of gratitude to Leslie, my colleagues at CBS and all the actors, producers and writers who were part of this incredible journey. I will always love this place, yet it is time for my next chapter, and I'm thrilled that I can pursue my other creative interests while still being part of the Company. I pass the baton to Glenn with great confidence and pride. He is an outstanding leader with great respect for the artistic process and a clear vision for programming success."

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