A timeline of the most important things to happen to ER

Season 1 begins, and will go on to win eight Emmys, including Juliana Margulies' for best supporting actress.

Jeanie Boulet (Gloria Reuben) learns she has HIV; many other regulars will face their own health crises.

ER gets its biggest audience ever — 47.8 million viewers — when it follows the Seinfeld finale.

George Clooney leaves the show, but will return for a surprise cameo in 2000 for Margulies' ER swan song.

Goran Visnjic joins the cast, eventually becoming the show's new romantic center with Maura Tierney.

Sally Field plays Tierney's mom, and wins an Emmy. Future Emmy bait: Ray Liotta and Don Cheadle.

Succumbing to a brain tumor, Dr. Greene (Anthony Edwards) is one of six hospital staffers to die thus far.

Season 10 begins: Parminder Nagra, Scott Grimes, and Linda Cardellini will join the crowded ER.

Last original cast member Noah Wyle leaves. Counting return cameos, he appeared in the most episodes: 254.

John Stamos joins the show, picking up the "hunky doctor" mantle once carried by Clooney and Visnjic.

After past castmembers returned to pay respects, the series ended on episode 332.