What interrupts concentration?

[WizardsBlade]

At first we thought it was any hit higher than your level, but with a closer look only a reaction or free action to someone using the concentrate action can break it. Is that right? What gets a reaction to concentrate?

[breithauptclan]

Readied actions.

[DM_Blake]

Disrupt a Spell is on page 196 and the action, Concentrate on a Spell, is on page 197.

So if anybody is casting a spell using verbal, somatic, or material component actions, or using one of their actions to Concentrate on a Spell, and they take damage during those exact actions but not between those actions (such as damaging my spellcaster on YOUR round even though he concentrated on his last round and will do so again on his next round).

To be more clear, all the spellcasting actions can be disrupted by damage AND concentrate actions can be disrupted too.

Damaging somebody during their own action requires a reaction or a free action.

Thinks like Attack of Opportunity won't work because Concentrate on a Spell is not one of the listed triggers.

However, the Ready action works just fine: "I ready to attack that mage if he concentrates on his spell". A perfectly valid trigger and according to the Ready rule (page 308), your attack becomes a reaction if your trigger occurs.

[EberronHoward]

The Mummy Pharaoh does explicitly get to use AoO on concentrate actions.

[Mats Öhrman]

DM_Blake wrote:

Thinks like Attack of Opportunity won't work because Concentrate on a Spell is not one of the listed triggers.

Disruptive, a 10th level Fighter feat, p. 93, allows the fighter to use an attack of opportunity against anyone who takes a concentration action.

Note that e.g. readying an action has the concentrate trait, so that feat may come as a nasty surprise to opponents in some situations.