For some TV shows, reaching the 100-episode mark is no big deal.

They just solve a crime each week. They can keep doing that forever.

But for "Lost" — which airs No. 100 on April 29 — this is huge. "There was a certain point where we would be sitting around going, 'Where do we go from here?' " said co-creator Damon Lindelof.

Many people were wondering that. In "Desperate Networks" (Doubleday, 2006), author Bill Carter says "Lost" was unpopular with most ABC and Disney executives. At one point, Disney boss Robert Iger reportedly called it "a colossal waste of time, energy and money."

And then that time and money paid off. "For me, the most gratifying thing has been the willingness of audiences to gravitate to an ambitious and creative show," said Carlton Cuse, who produces "Lost" with Lindelof.

Now "Lost" even has a predictable future. There will be 17 episodes this season, wrapping up on May 13; there will be 17 more next season, ending the series.

"It feels like a good number to us," Cuse said. "It's so difficult to do 23 or 25 episodes a year and do them well."

At least, it's difficult when you have a continuing story. For a while, that had fallen out of favor.

In the season before "Lost" debuted, there were nine dramas in the Nielsen top 20; with one exception ("ER"), all were crime-of-the-week shows.

Then "Lost" debuted in 2004. A plane crashed on a deserted island; strangers had to link.

ABC disliked an early script and brought in J.J. Abrams, the "Alias" and "Felicity" creator. He added a key notion — the island itself is a mysterious force — and added Lindelof.

Their pilot film scored well with test audiences and a once-reluctant network began pushing the show. "Lost" reached the Nielsen top 20 (at No. 13 and No. 18) its first two seasons.

One thing that helped is its unique structure. "Lost" doesn't just startle people at the end of each episode; it does it before each commercial break.

"The form of the show is that we have six act breaks," Lindelof said.

"We try to have something big at every break."

There's a more basic reason for the show's success: It's well-made, as the awards have shown.

In its first season, "Lost" won six Emmys, including best drama. In its first two, the Television Critics Association named it the best drama.

It got the top awards from the Writers Guild and the Screen Actors Guild; this month, it added the prestigious Peabody Award.

And once that first year was over, Lindelof said, there was time to plan an overview. There were "some incredibly detailed and lengthy discussions" of where the story was going.

That was hard to decide at first, because no one knew how many years "Lost" would last. "We were on cruise control," Cuse admits.

Then came the decision to do three final, limited-episode seasons.

"Lost" could take bigger steps. There had always been flashbacks, but now there were flash forwards and time travel.

And characters could transform. Most remarkable has been James "Sawyer" Ford.

"You say, 'Here's the bad boy. ... How can you change him in a way in which the audience won't cry "foul?" ' " Lindelof said.

That came with the time travel. Other characters were overwhelmed; Sawyer, who has spent his life scamming out of jams, adjusted easily and lied skillfully. He became a voice of reason.

All of this is complicated to follow and to plan. "We're constantly saying, 'Why did we decide to do time-travel?' " Lindelof said.

And most of it works. Somehow, "Lost" is looking fresh and vigorous at 100.