Caruso left after multiple contract disputes, so his cop John Kelly was transferred. Andy Sipowicz's new partner was Bobby Simone. The bond between Dennis Franz's Sipowicz and Jimmy Smits' Simone was much stronger — more buddy-buddy, even father-son — and viewers responded with affection for the pair and increased ratings. —Ken Tucker

Oh Shannen, if only you'd listened! After four seasons, Doherty left the original 90210 to be replaced by someone who was better at keeping the drama on screen. In 1994, Thiessen's Valerie Malone breathed new life (and bitchitude) into the series, which stayed on the air for another six years. —Maggie Pehanick

NewsRadio was a great ensemble comedy, but Hartman's egotistical news anchor Bill McNeal was unquestionably the center of the show. In the wake of the star's untimely death, the producers brought in Hartman's old Groundlings colleague (and fellow SNL performer) Lovitz as the new anchor. Lovitz was actually pretty funny, but the low-rated show only lasted one season post-Hartman. —Darren Franich

When Doug Ross left Chicago General for the Northwest, his longtime love Carol Hathaway wasn't the only thing left behind — there were also millions of heartbroken (mostly female) viewers. So what did the producers do when their classically handsome, emotionally damaged hunky doctor left? Well, they hired a classically handsome, emotionally damaged hunky doctor, of course. Ratings slid steadily after Clooney's departure and hit a low point during its 14th season (which, it should be noted, was still an average of about 9 million). —Sandra Gonzalez

The combination of from-day-one co-hosts Barbara Walters and Joy Behar has helped The View, despite two big recasts of its moderator, to remain stable. The fact is that even if Goldberg decides to move on, The View will continue — this is a show that's very smartly built around a concept (ladies gabbing about Hot Topics!) and not one singular personality. —Tanner Stransky

During Kilborn's fun and respectable hosting tenure, the show found smarmy comedy in human interest stories; under the shrewd, absurdist direction of Stewart, the focus shifted to news and politics. The result? Higher ratings and oodles of acclaim. In case you're counting, The Daily Show has won eight consecutive Emmys for Outstanding Variety, Music, or Comedy Program. —Dan Snierson

When it went off the air after nine seasons in 2002, The X-Files was considered the longest-running sci-fi show in broadcast TV history — but at a cost. Once Duchovny left the show as a full-time cast member in 2000 (explanation: His character was abducted by aliens), producers rejiggered the series, partnering costar Gillian Anderson with an FBI agent played by Patrick. For the show's final season, Annabeth Gish came on full-time as a potential replacement for Anderson. But viewers were disinterested in the new characters, and by the time both Duchovny and Anderson were reunited for the series finale, fans were hopelessly confused by the show's mythology. The X-Files has since been held up as an example of how not to finish a serialized drama. —James Hibberd

Ritter's character, overprotective dad and newspaper columnist Paul Hennessy, collapsed at the supermarket and died off screen. Sagal, who played his wife, Cate, took on a more central role afterwards, with James Garner joining the show as her surly father. Despite some powerfully dramatic episodes dealing with Paul's death, the show never fully recovered, in the ratings or creatively. —JA

Long was the first member of the heralded cast to win an Emmy, but after five seasons of playing the overeducated barmaid, Diane, she left the hit series to focus on a burgeoning movie career and her family. The writers sent Diane away to write her novel, stranding Sam (Ted Danson) on their wedding day. Alley's Rebecca arrived the next season, determined to run the bar after a rattled Sam had sold it. Though she initially seemed cold and aloof, she proved hopelessly insecure and corruptible. Before long, she was one of the Cheers gang, to her character's detriment and our weekly delight. Cheers remained a top 5 show for the next five seasons, and Alley eventually grabbed her own Emmy. —Jeff Labrecque

While Malina's Will Bailey was written as an awkward guy with some built-in friction with other characters when he took over Sam Seaborn's (Lowe) position as deputy communications director, it only served to emphasize the many differences between the two characters. But since the show had such a massively talented ensemble cast to share the screen time, which was the reason Lowe cited for leaving (though it was reportedly more about the money), the weight of its success or failure really didn't fall on Malina. —Abby West