HOLLYWOOD — The end is near once again for John Wells. It's staring him in the face.

The executive producer of one of television's groundbreaking shows sits with 45 pages of notes on his desk, a keyboard in his lap and a computer before him as he slowly taps out the final script to NBC's long-running medical drama "ER." While Wells knows generally how the show — set to air April 2 — will end, there are many details to work out — like exactly how George Clooney, who played Dr. Doug Ross for the first five seasons, will return.

"You want to try and find the essence of the series," said Wells, taking a break this month to discuss the conclusion to the program that has lasted for an astonishing 15 seasons. "You want to find the thing that people actually identify with in the series and do something that leaves them feeling satisfied for having spent X number of hours of their lives devoted to watching your ongoing narrative."

Wells, who has wrapped up other acclaimed television series as well — "The West Wing" and "China Beach" among them — is experiencing something of an anomaly, especially for network television. The wildly successful writer-producer actually gets to have an ending — but then only on some of his shows. Historically, for every big-bang omega moment accorded to a "MASH," "Friends" or "Seinfeld," there are hundreds more programs crowding television's graveyard that came to their final resting place with a whimper.

But thanks largely to the ascent of cable television and its increasingly sophisticated storytelling, planned endings for television programs are slowly becoming more common. As shows have become more literary in their approach by featuring complex characters, expectations have risen for an appropriate farewell. While audiences may not have cared about what ultimately happened to the characters on "Jake and the Fatman," they certainly do care about the stranded, tortured souls on "Lost," the finale of which has been slated for 2010.

"In the '70s and '80s, the characters basically did the same kind of things and they didn't change that much," said David Chase, creator of HBO's "The Sopranos," whose television-writing roots reach back to "The Rockford Files." "The audience didn't really know about the characters' beginning and they really didn't care about the end either.

"But today the way people view a television series has changed," added Chase. "You're kind of in the ring now with Charles Dickens and Norman Mailer or something. Maybe it means the whole thing is richer and deeper. You've raised thoughts and issues that need to be dealt with."

But just because an audience wants a fitting end to its favorite show doesn't mean it is going to get one. Ask the fans of Aaron Sorkin's ill-fated dramedy "Studio 60 on the Sunset Strip." Economic pressures, particularly on the major networks, still often work against providing closure to most series.

By the time the networks announce their fall schedules, usually in mid-May, most comedies and dramas have long since wrapped for the season. So the studio may not know in time that a finale is needed. And then there are the raft of shows whose lifelines are cut midseason because of disappointing ratings — the reason why "Twin Peaks" FBI agent Dale Cooper is still trapped in the Black Lodge and his doppelganger is on the loose.

The growing demand for endings can be seen in the clamor for other mediums to complete the work begun, but cut short, on television. Naturally, like the fans of HBO's hard-bitten western "Deadwood," most invest their hopes in the cinema to finish the story line. And Bryan Fuller of ABC's recently canceled "Pushing Daisies" has toyed with continuing his dramedy in comic book form.

"Why should television be the only art form that doesn't routinely have an ending?" asked Shawn Ryan, creator of FX's "The Shield," which recently aired its series finale after seven seasons. "A song has an ending, a painting has borders, a movie has an ending, a book has an ending, but unfortunately television series because of their economic nature get interrupted."

But even if the audience gets its ending, it may not be the one it wants — or thinks it wants. Few know this better than Chase, whose now famous — or infamous — 10-second blackout in the final moments of "The Sopranos" created an uproar.

Chase's reception hints at what writers intimately know — endings, at least good ones, are exceedingly difficult, especially in television. Compared with movies, with their concentrated focus and mere two hours of screen time, television endings must take into account the medium's sprawling nature, which has far more characters and scores of previous hours of material.

"An ending should bring your series full circle and should reveal something about every episode that has come before it," said Ryan. "It should give a sense of completeness — that's what we aimed for, and we may have not achieved all that, but I think it's important to strive for."