THE COMPLETE LIST: Leroy Jethro Gibbs has rules that he enforces with everyone on his team, in 3x05 "Switch" he told Ziva that there are approximately fifty rules. However, not all of the rules have been revealed and some numbers have two different rules. Here are the rules that have been revealed so far:
Rule #1: Never let suspects stay together. (Franks) 1x01 'Yankee White'
Rule #1: Never screw over your partner. 4x14 'Blowback'
Rule #2: Always wear gloves at a crime scene. (Franks) 1x01 'Yankee White'
Rule #3: Don't believe what you're told. Double check. (Franks) 1x01 'Yankee White'
Rule #3: Never be unreachable. 3x13 'Deception'
Rule #4: The best way to keep a secret? Keep it to yourself .Second best? Tell one other person - if you must. There is no third best. 4x11 'Blowback'
Rule #5: You don't waste good. (…"You're Good" Gibbs says to DiNozzo) 8x22 'Baltimore'
Rule #6: Never apologize — It’s a sign of weakness. A rule used more often than the others, but finally given a number in 7x12 'Flesh and Blood' during a great Gibbs/DiNozzo moment. The first person to use that phrase was John Wayne in, "She Wore a Yellow Ribbon." "Never apologize, mister, it. It's a sign of weakness." — In the episode 3x23 'Hiatus Part-1' DiNozzo explains to NCIS Director, Jenny Shepard, that Gibbs took this one from "The Duke himself."
Rule #7: Always be specific when you lie. 1x23 'Reveille'
Rule #8: Never take anything for granted. 3x10 'Probie'
Rule #9: Never go anywhere without a knife. 1x13 'One Shot, One Kill' & 1x20 'Missing'
Rule #10: Never get personally involved on a case. (Said by Tony, agreed on by Gibbs) 7x21 'Obsession'
Rule #11: When the job is done, walk away. 6x24 'Semper Fidelis'
Rule #12: Never date a coworker. 1x15 'Enigma'
Rule #13: Never, ever involve a lawyer. 6x07 'Collateral Damage' & 7x24 'Rule Fifty-One'
Rule #14: Bend the line, don't break it. And Gibbs said human smuggling breaks it.
Rule #15: Always work as a team. 5x05 'Leap of Faith'
Rule #16: If someone thinks they have the upper-hand, …break it! (DiNozzo) 8x24 'Pyramid'
Rule #18: It's better to seek forgiveness than ask permission. 3x04 'Silver War'
Rule #22: Never, ever bother Gibbs in interrogation. 4x10 'Smoked'
Rule #23: Never mess with a Marine's coffee if you want to live. 2x09 'Forced Entry'
Rule #27: Two ways to follow: First way, they never notice you, — second way, they only notice you. 7x15 'Jack Knife' & 7x24 'Rule Fifty-One'
Rule #35: Always watch the watchers. 8x22 'Baltimore'
Rule #36: If you think you're being played, you are
Rule #38: Your case, your lead. 6x16 'Bounce'
Rule #39: There is no such thing as coincidence. 7x21 'Obsession'
"The 40's are for Emergency Use Only!" (Mentioned by Abby) 7x24 'Rule Fifty-One'
Rule #40: If it seems someone is out to get you, they are. (Said by Abby, agreed on by Gibbs) 7x22 'Borderland' & 7x24 'Rule Fifty-One'
Rule #42: Don't ever accept an apology from someone who just sucker-punched you
Rule #44: First things first. Hide the women and children. 7x23 'Patriot Down'
Rule #45: Clean up your own mess. (Gibbs Said, "Left a mess I've got to clean up." — DiNozzo commented that's basically Rule #45) 7x24 'Rule Fifty-One' (See Below)
Rule #51: Sometimes — You're Wrong! 7x24 'Rule Fifty-One'
Rule #69: "Always carry a lighter (Marine Corps Zippo)." I believe the Quote attributed to #69 above was said to Tobias by Gibbs during the episode involving former wife Diane and the disappearance of her current husband. It was "Never trust a woman who doesn't trust her man."