Lucy Knight is a fictional character from the NBC television series ER, portrayed by actress Kellie Martin. The character was part of the show for the fifth and sixth seasons. When she was first introduced on ER, Dr. Doug Ross called her a "by the book" medical student. Kellie Martin's image was removed from the main cast opening credits in the 15th episode of season 6.

Prior to this role in ER, Martin was nicknamed "One Take Kellie". She was not used to the medical jargon her character had to say, however, and claimed that it once took 12 takes to correctly pronounce "renal vein thrombosis."

During season 5 Lucy is a third-year medical student. Dr. John Carter is quickly assigned as her resident while she completes her emergency room rotation. Knight and Carter vary between respectful and contentious interactions.

At the beginning of her rotation at County General, Lucy is popular with the attendings and nurses. Lucy encounters conflict with Nurse Carol Hathaway after she asks for help with an IV. Lucy, unable to start an IV asks Carol for help and does not admit to Carter that she cannot start one until they wind up in a trauma situation. Carter is furious at her for lying to him and says if she ever does so again she will be off his rotation permanently. Carter again is angry with her after a Halloween party at her med school dormitory spirals out of control and two students nearly die. Angry about work that has been thrown at him at the last minute, Carter leaves Lucy in charge of the party even though, as Resident Assistant (RA) at the dormitory, this is his responsibility. Carter blames Lucy for him losing his RA position and they fight again when Carter overrules her on a case where Lucy's judgment was correct. He then accidentally hears a recording on Lucy's Palm Pilot in which she describes his condescending treatment and is again upset.

Carter and Lucy eventually reconcile after the episode "The Good Fight" when the pair spend a day and evening searching for a little girl's father who has the same extremely rare blood type as she does. Although the pair find the father, the girl's condition remains critical and the episode does not clarify whether the blood donation came in time to help save her. During this episode, Lucy reveals to Carter that she was brought up by her mother, and that she never knew her father.

During the episode The Storm: Part 1, Lucy accidentally kicks Carter while performing a Tae Bo work-out video with a patient in an exam room; he falls and suffers a cut on his head. She later notices Carter secretly admiring her backside and the pair make out in the x-ray room after she treats his cut, but Carter quickly stops this from progressing to sex saying romantic involvement between residents and medical students is unethical. Lucy's following rotation in the psychology department goes exceptionally well, with the senior doctors there pleased with her results. This is the first time the show has a main character who starts out working in the ER but is shown to be a poor match for emergency medicine and a good match instead for a different medical field (a similar, longer path is traced starting in Season 10 for Neela Rasgotra, who is an uneven ER doctor but emerges over time as a superb surgeon).

The near-end of Season 5 reveals Lucy has been prescribed Ritalin to treat ADD since high school. Carter urges her to give up the drug and she briefly complies, however, when her performance at County begins to suffer she resumes taking it. When Carter finds out and chides her she bluntly says that she does not expect him to understand or care, since "it's clear I've never lived up to your expectations."

Lucy's role in the early episodes of season 6 is smaller than season 5 with minor sub plots, such as helping a young artist who is abusing cocaine and urging Dr. Dave Malucci to come forward and admit he may have caused an explosion in the ER by possibly forgetting to turn off a bunsen burner.