George Clooney's exit, Dr. Greene's death, and John Stamos' entrance are major moments in the series' history

By Abby West
Updated March 06, 2009 at 05:00 AM EST

September 19, 1994
Season 1 begins, and will go on to win eight Emmys, including Margulies’ for best supporting actress.

September 26, 1996
Jeanie Boulet (Gloria Reuben) learns she has HIV; many other regulars will face their own health crises.

May 14, 1998
ER gets its biggest audience ever — 47.8 million viewers — when it follows the Seinfeld finale.

February 18, 1999
Clooney leaves the show, but will return for a surprise cameo in 2000 for Margulies’ ER swan song.

September 30, 1999
Goran Visnjic joins the cast, eventually becoming the show’s new romantic center with Maura Tierney.

November 16, 2000
Sally Field plays Tierney’s mom, and wins an Emmy. Future Emmy bait: Ray Liotta, Don Cheadle.

May 9, 2002
Succumbing to a brain tumor, Dr. Greene (Edwards) is one of six hospital staffers to die…so far.

September 25, 2003
Season 10 begins: Parminder Nagra, Scott Grimes, and Linda Cardellini will join the crowded ER.

February 26, 2004
A man threatens the ER in a tank. It has suffered worse: a smallpox scare, explosions, and a shooting spree.

May 19, 2005
Last original cast member Wyle leaves. Counting return cameos, he’s appeared in the most episodes: 254.

November 10, 2005
John Stamos joins the show, picking up the ”hunky doctor” mantle once carried by Clooney and Visnjic.

April 2, 2009
After past cast members return to pay respects, the series ends on episode No. 332.