In general, if an action wouldn’t normally provoke an attack of opportunity, the character need not make a Concentration check to avoid being distracted. If the Concentration check succeeds, the character continues with the action (such as casting a spell, concentrating on an active spell, using a spell-like ability or skill); if it fails, the action fails and is wasted. The table below summarizes various types of distractions that require Concentration checks. If the distraction occurs while a character is trying to cast a apell, add the spell’s level to the Concentration DC. For multiple distractions, make a separate check for each one; any failed check means the action fails. Ability damage does not cause loss of Concentration unless then affected ability is Stamina, in which case the number of hit points lost due to the reduction in Stamina counts as damage taken.

Concentration DC* Distraction
10 + Damage dealt Damaged during the action.**
10 + half of continuous Taking continuous damage during the damage dealt last action.
Distracting spell's save DC

Distracted by non damaging  spell, or weather casued by spell (such as whirldwind).

10 Vigorous motion (e.g., on a moving mount)
15 Violent motion (e.g., on a galloping horse).
20 Extraordinary violent motion (e.g., an earthquake).
15 Entangled
20 Grappling or pinned
*If distracted while trying to cast, concentrate on or direct a spell, add spell's level to the DC
**During the casting of a spell with a casting time of 1 round or more, or an activity that takes more than a single full-round action.
† If the spell allows no save, use the save DC if it did a allow a save.

Action:None. A Concentration check is either a free action (when a reaction) or part of another action (when attempted actively).

Try Again:Yes, though a success doesn’t cancel the effects of a previous failure. 

Special:Concentration can be used to cast a spell, use a spell-like ability, or use a skill defensively and avoid attacks of opportunity. The check’s DC is 15 (plus the spell’s level, if applicable). If the check succeeds, the action can be attempted normally; if it fails, the related action automatically fails and is wasted.  

Characters with the Combat Casting feat receive a +4 bonus on Concentration checks when casting spells or using spell-like abilities while on the defensive or while grappling or pinned. 

High elves receive a +2 racial bonus on Concentration checks. Concentration is a class skill for all high elf characters.