A duel is an engagement in personal combat between two characters.
Duel for honor
Rulers have access to a Duel interaction if they have 
War focus, 
membership in a Warrior Lodge, or a bloodline that "Enables dueling".
Unless the dueling game rule is set to "Unrestricted", the initiator must have one of the following reasons to challenge the target:
- Rivalry
- Foe (certain opinion modifiers toward the initiator, including "Cuckolded" and "Attempted Murder")
- Target is Excommunicated
; initiator is 
and same religion - Target is Decadent
; initiator is and Muslim - Initiator has a mission to duel target (Warrior Lodge quest)
Both characters must be in good shape to duel:
- Duelers are NOT inaccessible
- Duelers are NOT pregnant
- Duelers have NOT recently been in any duel (within 3 months)
- Duelers do NOT have the character modifier Bedridden or Severe Illness
- Duelers must each be at peace (for unlanded characters, this means that their liege must be at peace)
Strong Claim duel
With the Holy Fury DLC, a tribal ruler may duel another tribal ruler over a strong claim. Both rulers must be independent or in the same realm. Many of the other requirements and mechanics are the same as in challenge duels, as listed above.
If you lose a Strong Claim duel and die, and the winner is a member of your dynasty, you may switch play to the other character. This makes Strong Claim duels a potentially effective way to consolidate titles after a gavelkind split, at the price of culling dynasts.
If you are the current title holder, you have the choice of rejecting the duel, at the cost of getting 
.
Other duels
- Battlefield duels
Germanic men may get an event suggesting they challenge a disliked character to a Holmgang. Opinion must be below -40. Possible opponents are the liege, any vassal, or any fellow vassal.- Confronting a spouse's lover may result in a duel.
As a method of suicide, you may challenge a rival to a duel that you intend to lose. Unlike other methods of suicide, this is considered an honorable death.- When attempting to establish a warrior lodge bloodline, you will face a series of increasingly difficult duels.
Some event duels use different code to determine the outcome, but combat_rating_diff is generally a major factor.