Beware of spoilers!!!

Season One: September 19, 1994 - May 18, 1995

Mark deals with his failing marriage to Jen.
Carol recovers from her suicide attempt & picks up the pieces.
Carter attempts to adjust the face pace life of the ER.
Benton tries to juggle caring for his mother & his busy schedule.
Ross struggles that Carol has moved on with her life.
Susan deals with her pregnant sister, Chloe.

Season Two: September 21, 1995 - May 16, 1996

Mark deals with his family moving to Milwaukee & the failure of his marriage. Also is promoted to attending physician.
Doug treats an HIV-positive child against the wishes of the senior staff, saves a child from a sewer, & reconnects with his father, briefly.
Susan is left with Little Suzy after Chloe skips town. Chloe eventually comes back and moves to Phoenix with the baby.
Carter earns a place as a surgical intern.
Carol & Shep, a firefighter, move in together but their relationship falls apart after Shep's partner, Raul, is burned and dies.
Benton is dating Jeanie but they break up.
Jeanie finds out she may be HIV-positive after her ex-husband, Al, is diagnosed.

Season Three: September 26, 1996 - May 15, 1997

Susan leaves for Phoenix to be with Chloe and Little Suzy.
Mark is upset that Susan left since he declared he had feelings for her and he develops depression. He is attacked in the ER bathroom which causes him to be extremely paranoid.
Carol & Doug draw closer as Doug tries to work on himself. Carol considers medical school but decides she loves nursing too much.
Carter is now an intern & often fights with Benton.
Dennis Gant, a fellow intern, ends up committing suicide which devastates Carter.
Benton starts to date Carla Reece who becomes pregnant and has their son, Reese, prematurely.
Weaver supports Jeanie when she is forced to reveal her HIV-status when Mark sees her medical records.

Season Four: September 25, 1997 - May 14, 1998

The live episode 'Ambush' airs.
Dr. Morgenstern, head of the ER, has a heart attack so Weaver steps in.
Two new doctors join the ER - Anna Del Amico & Elizabeth Corday.
Corday is in the USA under a fellowship with Dr. Romano but she rejects his advances and he decides not to renew the fellowship.
Mark & Doug take a road trip to California to bury Doug's dad. On the same trip, Mark finds out his mother is sick.
Mark dates the new desk clerk, Cynthia.
Carol & Ross become engaged but Carol can not commit just yet.
Carol opens the free clinic with the help of Carter's grandmother.
Jeanie is fired and believes it is become of her HIV-status. She is hired back by Anspaugh and he eventually has remorse over the firing after she helps treat his dying son.
Benton and Corday date.
Carter's cousin, Chase, has a drug problem and Anna tries to help him.

Season Five: September 24, 1998 - May 20, 1999

Lucy Knight does her ER rotation under Carter.
Corday is forced to start her internship over again to keep her job and ends it with Benton.
Benton's son is hear impaired.
Weaver is passed over for the full-time chief.
Ross once again breaks protocol and tells a mother how to give medication to her disabled son, which she uses to kill her son. Ross resigns and moves to Seattle.

Season Six: September 30, 1999 - May 18, 2000

Carol finds out she is pregnant with Doug's baby. She has twins, Tess & Kate, and moves to Seattle to be with Doug.
Lucy & Carter are attacked by a psychotic patient. Lucy dies.
There is an intervention after Carter develops an addictions to pain medication and Benton takes him to rehab.
Several new doctors join the ER - Dr. Kovac, Dr. Finch, Dr. Malucci, & Dr. Chen.
Jeanie leaves to care for her HIV-positive child.
Abby Lockhart begins her 3rd year medical student rotation.
Mark and Elizabeth begin dating and Mark deals with his father's passing.

Season Seven: October 12, 2000 - May 17, 2001

The ER gets a re-model.
Carter completes rehab.
Abby is forced to drop out of medical school, starts dating Luka, and has to deal with her mentally ill mother.
Mark has terminal cancer which he keeps a secret.
Mark & Elizabeth marry & welcome daughter, Ella.
This season marked the 150th episode.

Season Eight: September 27, 2001 - May 16, 2002

Susan Lewis returns.
Mark deals with his rebellious teenage daughter.
Benton & Cleo leave
Michael Gallant & Greg Pratt join the staff.
Weaver starts to date Sandy Lopez
Mark Greene passes away.

Season Nine: September 26, 2002 - May 15, 2003

Elizabeth leaves Chicago for England but returns.
Carter becomes the central character.
Romano loses his arm in an accident.
Luka and Carter go on a relief mission to Africa.
Abby continues to have family troubles with both her mother and brother.
Pratt and Gallant clash.
Pratt and Chen date.
Carter proposes to Abby.

Season Ten: September, 2003 - May 13, 2004

Abby returns to medical school.
Luka is feared to be dead in Africa.
Romano is killed in an accident.
Gallant is deployed to Iraq.
New staff join the ER - Neela Rasgotra, Archie Morris, & Samantha Taggart.
Chen struggles with her ailing father.
Luka & Sam grow close. Luka connects with her son, Alex.
Neela and Abby do their rotation in the NICU where Kerry & Sandy's son, Henry is.
Weaver's partner, Sandy, dies after being injured on the job.
Susan is pregnant.
Carter dates Kem. She gives birth to their son, who is born stillborn.

Season Eleven: September 23, 2004 - May 19, 2005

Corday leaves after an illegal surgery and returns to England.
Chen leaves after helping her ailing father die and returns his body to China.
Carter leaves to be with Kem in Africa.
Weaver meets her biological mother, who is a patient in the ER.
Abby is now an intern as is newbie, Ray Barnett.
Neela comes back to county after another intern resigns.
Morris is appointed the new chief resident.
Sam has a pregnancy scare and learns that her ex has been in jail.

Season Twelve: September 22, 2005 - May 18, 2006

After Susan leaves, this is the first season to not feature any of the original cast.
Luka & Sam break things off.
Luka & Abby get back together & they learn they are expecting.
Weaver has hip replacement surgery and no longer needs her crutch.
Pratt joins Carter in Africa.
Sam's ex returns and kidnaps Sam & Alex and has a shoot out with the police in the ER.
Neela and Gallant are married.
Gallant is killed in Iraq.
Luka is the new chief of the ER.

Season Thirteen: September 21, 2006 - May 17, 2007

This season started the different theme music/opening credits.
Abby has her son, Joe, prematurely.
Sam & her son suffer the effects from the trauma with her ex.
Weaver leaves when Luka is forced to make budget cuts.
Ray has an accident that causes him to lose his legs & her returns to Baton Rouge.
ER's newest intern is Tony Gates, a paramedic.
Luka is sued and stalked by Curtis Ames.
Both Ray and Tony pursue Neela.
Abby & Luka marry.
Luka leaves to care for his father in Croatia.

Season Fourteen: September 27, 2007 - May 15, 2008

Short season due to the writer's strike (19 episodes).
This season had the 300th episode.
Moretti, the new chief of the ER, has a challenging time trying to make his authority known.
Abby struggles with being a single mother with Luka away & begins to drink again.
Morris and Pratt pass their boards.
Sam & Tony date.
Abby joins Luka in Croatia after his father dies & they make plans for their future.

Season Fifteen: September 25, 2008 - April 2, 2009

Final season.
Pratt dies as a result of injuries from an ambulance explosion.
Cate Banfield is the new chief of the ER.
Abby & Luka leave for a new life in Boston.
Carter gets a kidney transplant.
Sam's son is injured in a car crash. This causes Sam and Gates to break up.
Neela struggles professionally and personally.
A new medical facility is opened by Carter and his old colleagues return to support him.