Ask an NCIS fan for a Gibbs rule and you’ll get one in about two seconds.
Rule #9. Rule #39. Rule #51. Most of us can quote them. Almost nobody can say where they first heard them.
So here’s the list: the rules fans actually remember, in Gibbs’ own wording, with the episode each one first appeared in. Some of them contradict each other. That’s part of the fun.
Also read: Mark Harmon’s father and the code behind Gibbs.
Where the Rules Came From
They didn’t come from the Marines. In Season 1’s “Missing” (2004), Tony guessed the rules came from the Corps, and Gunnery Sergeant Bill Atlas said he’d never heard of them, per the NCIS wiki.
The real answer came in Season 6’s “Heartland” (October 14, 2008), when we see Gibbs meet Shannon at a train station in 1976. She tells him, “Everyone needs a code they can live by.” The only rule of hers we ever hear is “Never date a lumberjack.”
Gibbs later wrote his own rules down and kept them in a small box at home. We see it in the Season 7 finale, “Rule Fifty-One” (May 25, 2010).
Mark Harmon has talked about that origin. In 2012 he told TV Equals, as quoted by Looper: “I think they’ve established that the rule thing came from his wife, Shannon, and yet, you know what, maybe rules aren’t a bad thing for people.”
The Rules Fans Quote Most
Rule #1: “Never let suspects stay together.”
Season 1, Episode 1, “Yankee White” (September 23, 2003). Gibbs says it to Kate on Air Force One in the very first episode. He breaks it on purpose in “Ships in the Night” (Season 8, 2011) to get a confession.
Rule #1, again: “Never screw over your partner.”
Season 4, Episode 14, “Blowback” (February 6, 2007). Jenny Shepard throws it back at him. Yes, that’s a second Rule #1. More on that below.
Rule #3: “Don’t believe what you’re told. Double check.”
Season 1, Episode 1, “Yankee White.” Gibbs tells Kate this one in the same episode, along with Rule #2: “Always wear gloves at a crime scene.”
Rule #3, again: “Never be unreachable.”
Season 3, Episode 13, “Deception” (January 17, 2006). Same number, different rule.
Rule #5: “You don’t waste good.”
Season 8, Episode 22, “Baltimore” (May 3, 2011). Gibbs says it to Tony in the episode about how the two first met.
Rule #6: “Never say you’re sorry. It’s a sign of weakness.”
Season 7, Episode 12, “Flesh and Blood” (January 12, 2010). Gibbs had said it for years, but this is when it got its number. It’s a nod to John Wayne’s “Never apologize, mister, it’s a sign of weakness,” and Tony calls that out in “Hiatus, Part 1.” Gibbs breaks it himself too, at least eight times by the wiki’s count.
Rule #9: “Never go anywhere without a knife.”
Season 1, Episode 13, “One Shot, One Kill” (February 10, 2004). It’s Mark Harmon’s favorite. More on that below.
Rule #12: “Never date a coworker.”
Season 1, Episode 15, “Enigma” (February 24, 2004). Gibbs broke it himself, with Jenny Shepard.
Rule #18: “It’s better to seek forgiveness than ask permission.”
Season 3, Episode 4, “Silver War” (October 11, 2005). Half of Gibbs’ career in one line.
Rule #23: “Never mess with a Marine’s coffee… if you want to live.”
Season 2, Episode 9, “Forced Entry” (December 7, 2004). The one with the best threat attached.
Rule #39: “There is no such thing as coincidence.”
Season 7, Episode 21, “Obsession” (May 4, 2010). That’s its first numbered appearance, but the idea shows up earlier. In Season 3’s “Head Case,” Tony says, “We don’t believe in coincidences around here,” and Gibbs adds, “However, we do believe in bad luck.”
Rule #45: “Clean up the mess that you make.”
Season 7, Episode 24, “Rule Fifty-One.” An earlier version shows up in “Hiatus, Part 2”: “Never leave behind loose ends.”
Rule #51: “Sometimes you’re wrong.”
Season 7, Episode 24, “Rule Fifty-One.” We watch Gibbs write this one. He flips over the card for Rule #13 (“Never, ever involve lawyers”) and writes #51 on the back. The episode is named after it.
Rule #91: “When you decide to walk away, don’t look back.”
Season 18, Episode 16, “Rule 91” (May 25, 2021). Gibbs paints it on his boat. When Bishop asks what it means, he says, “You’re living it,” per TV Insider.
Why There Are Two Rule #1s and Two Rule #3s
Fans noticed early. Even McGee points out in “Ships in the Night” that Rule #1 has been taken twice.
Executive producer Shane Brennan explained it. When Gibbs joined NIS, Mike Franks told him he didn’t need dozens of rules, just three golden ones. “Three of them are Gibbs’ rules; three of them are Mike Franks’ rules,” Brennan said, as reposted by NCISfanatic. He left fans to guess which were whose.
What Mark Harmon Says
In May 2026, Harmon joined the official NCIS: Partners & Probies podcast and was asked for his favorite rule. He pulled a Swiss Army knife out of his pocket.
“‘Carry a knife’ is a good one for me,” he said, per TVLine. “You’d be surprised how useful it is.”
He also admitted airport security has taken his pocketknife before.
The Rule That Ended It
Gibbs painted Rule #91 on his boat in the Season 18 finale. Then he lived it.
In Season 19’s “Great Wide Open” (October 11, 2021), Gibbs ends up fishing with McGee in Alaska and tells him, “I’m not going back, Tim. I’m not going back home,” per Deadline. Cheat Sheet reports that the writers built that final episode around Rule #91.
And the Rules Keep Coming
Gibbs is gone from the main show, but new rules still turn up. “Stolen Moments,” a Season 23 episode from December 9, 2025, gave us Rule #24: “There is no such thing as perfect timing.”
Which one do you live by?
