Leroy Jethro Gibbs has rules that he enforces with everyone on his
team, in "Silver War" he told Ziva that there are approximately fifty
rules. However, only a few rules have been revealed. Here are the rules that have been revealed so far:
Rule #1: Never let suspects stay together.
Rule #1: Never screw over your partner.
Rule #2: Always wear gloves at a crime scene.
Rule #3: Don't believe what you're told. Double check.
Rule #3: Never be unreachable.
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.
Rule #8: Never take anything for granted.
Rule #9: Never go anywhere without a knife.
Rule #11: When the job is done, walk away.
Rule #12: Never date a coworker.
Rule #13: Never, ever involve a lawyer.
Rule #15: Always work as a team.
Rule #18: It's better to seek forgiveness than ask permission.
Rule #22: Never, ever bother Gibbs in interrogation.
Rule #23: Never mess with a Marine's coffee if you want to live.
Rule #38: Your case, your lead.
Rule #39: There is no such thing as coincidence.
Another
one of Gibbs' rules which is used more often than the others, but
without a number, is "Never apologize — Its a sign of weakness."

