Endings are important. A great ending to a movie, book or TV show can sometimes be the difference between it being considered just plain good, and being forever memorable. Would "Casablanca" be remembered as one of the greatest films of all time if Ingrid Bergman hadn't gotten on that plane? Would "Thelma and Louise" be as remembered if they hadn't hit the gas and gone off that cliff?

In the world of television shows, the final episode is often its highest rated, and producers frequently take the opportunity of the large audience to surprise viewers with twists and turns even their wildest imaginations could not have conceived. Here is a look at some of the most memorable television series finales of the last 30 years.

'Cheers'

NBC's "Cheers" was one of the most popular and beloved sitcoms off all time, and it went out with a bang in 1993 following an 11-season run by pulling in over 80 million viewers, making it the second-highest rated finale ever.

In a 98-minute episode, the show saw the return of Sam's old love-interest and co-star, Diane Chambers (Shelly Long), who had left after season five. The show ended on a subtle, bittersweet note, with main character Sam Malone (Ted Danson) realizing, thanks to the help of his barfly customer Norm Peterson (George Wendt), that his true love was not Diane, nor any other woman, but the bar Cheers itself.

'St. Elsewhere'

TV producers often take the finale of a series to break the mold of a show and spin audience's perceptions of it in an entirely new direction. NBC's "St. Elsewhere" did this, and then some, with its provocative finale in 1988.

After following the inner-workings of St. Eligius Hospital in Boston for six seasons, audiences were left with the image of a young autistic boy staring into a snow globe with a model of the St. Eligius Hospital inside of it, suggesting the entire show had taken place inside the boy's mind.

'Six Feet Under'

"Six Feet Under," the HBO show that ended in 2005 and explored the lives of two brothers who ran a funeral home, chose to break with the convention of the show in its final scenes and throw the audience a curve ball it was not expecting.

The show had always played out in a slow, chronological order, but in its final scenes it flashed forward in time, showing how each main character was going to meet their demise. Many shows choose ambiguity in their final episodes, but "Six Feet Under" left no question as to the fate of its characters. But somehow, by focusing on death, the final scenes became a celebration of the characters' lives, happy or not, and reminded the audience to enjoy their own lives as much as they can, while they still can.

'Mary Tyler Moore'

CBS's Mary Tyler Moore" was one of the most critically-acclaimed shows of all time, and ended its seven-year run in 1977 at the top its game after winning the Emmy for Outstanding Comedy Series its last three seasons. The final episode saw the crew at Minneapolis' WJM-TV all laid off due to low ratings - everyone but the anchor, Ted Baxter (Ted Knight), who was the one responsible for the low ratings. In a classic sendoff, Mary and the other main characters left the studio together singing the lines of "It's a Long Way to Tipperary."

'Newhart'

After starring in the successful "Bob Newhart Show" in the 1970s, comedian Bob Newhart returned to the CBS airwaves in 1982 in "Newhart," where he played a New Yorker who moved to Vermont to run an inn in a town populated with oddball characters.

On the final episode in 1990, Newhart's character awoke from a nightmare, lying in bed next to Suzanne Pleshette, who played his wife on the old "Bob Newhart Show." The new show had all been a bad dream brought on by eating too much Japanese food before Bob went to bed. It was the ultimate inside joke, and a reward to fans that had followed Newhart's career throughout the years. The final credits were played over the music from the old "Bob Newhart Show."

'Homicide: Life on the Street'

Critics in the 1990's loved NBC's "Homicide" and hailed it as the most realistic and gritty crime drama ever produced for the small screen. Set in the homicide unit of the Baltimore Police Department, the show was a far cry from the typical cops-and-robbers shows of the past, for the cops weren't always the good guys on "Homicide," and were often less than heroic.

The series' final episode explored that very issue. In the pilot episode, the show's closest thing to a main character, Detective Tim Bayliss (Kyle Secor), joined the homicide unit as a young, by-the-book, idealistic cop looking to make a difference out on the street. By the series finale seven years later, Bayliss had become hard, cynical, and no longer believed in the system he had spent his adult life serving.

While it wasn't 100 percent clear, the audience was left with the impression that Bayliss had committed a vigilante-style murder, and had become the very thing he had spent his career fighting. It was a sobering message: If Bayliss no longer believed in the system, how could the audience?

'Buffy the Vampire Slayer'

Every day for seven seasons on the WB and UPN, Buffy Anne Summers (Sarah-Michelle Gellar) woke up knowing it would be another long day battling the forces of evil.

It was a burden that weighed heavy upon her, and in the series finale in 2003, Buffy finally entered the gates of hell for an epic battle that left Buffy and her army of slayers victorious but destroyed the entire town of Sunnydale, where the series had taken place. As Buffy sat at the edge of a large crater, looking over the destruction, her companions turned to her, asking, "What do we do now?"

Slowly, a smile formed on Buffy's face, and the show ended before any answer was given. But the answer was not important. What was important was that for the first time, Buffy had a choice in the matter.

'Everybody Loves Raymond'

CBS's "Everybody Loves Raymond" went off the air in 2005 after a nine-year run, and had the highest ratings of any sitcom on the air for its last few seasons. What made this episode so memorable was its lack of trying to be memorable.

After previous sitcom champs "Cheers" and "Seinfeld" went out with overblown hour-plus finales, "Raymond" took a simpler, subtler approach, with a regular 22-minute episode that ended with the Barone family sitting down to dinner. Only the long fade to black indicated that it was a final episode. It was a classy end to a very classy show.

'M*A*S*H'

Based on the 1968 novel and 1970 film of the same title, "M*A*S*H" ended its 11-year run on CBS in 1983, and the series finale is still the most-watched program in United States television history, pulling in an estimated 50 million households. The show itself lasted twice as long as the Korean War it depicted, and in the finale the war finally came to an end for Capt. "Hawkeye" Pierce (Alan Alda) and the rest of the 4077th Mobile Army Surgical Hospital.

'The Sopranos'

Viewers who had followed the murders, mayhem and middle-aged angst of Tony Soprano wanted, and expected, closure when the HBO show went off the air earlier this year.

But when the screen cut to black just as it seemed Tony was about to meet his well-deserved demise, many fans were outraged and felt like they had been ripped off. But the ending raised some interesting questions, not just about Tony Soprano, but about its viewers. Why did viewers expect, even demand, closure from a show that swam neck-deep in moral ambiguity? Tony was a life-long criminal, adulterer and murderer - yet audiences liked him.

Maybe he was liked because, despite his evil tendencies, everyone saw a little of themselves in the New Jersey mafia boss. To see Tony Soprano (James Gandolfini) get what he deserved might have helped the audience distance themselves from him, to tell themselves they really weren't like Tony and really weren't going to end up like him.

But when he did end up just like everyone else - adrift in life despite his best efforts to figure this world out, munching on onion rings - it was a disturbing, mind-boggling and unforgettable ending to a show that explored the complexities of the human condition and, like life itself, gave no clear answers to the troubles of the soul.

Other Notable Finales

• 'Dallas'
• 'The Tonight Show with Johnny Carson'
• 'Late Night With David Letterman'
• 'Seinfeld'
• 'The Wonder Years'
• 'The West Wing'
• 'Mad About You'
• 'Arrested Development'
• 'The Cosby Show'
• 'Happy Days'
• 'All In The Family'
• 'Friends'
• 'Magnum P.I.'
• 'Quantum Leap'
• 'Star Trek: The Next Generation'
• 'Angel'