Check: A character makes a Concentration check whenever he or she may potentially be distracted (by taking damage, by harsh weather, and so on) while engaged in some action that requires the character's full attention. Such actions include using skills that provoke attacks of opportunity while in a threatened square. In general, if an action would not normally incur an attack of opportunity, a character doesn't need to make a Concentration check to avoid being distracted.

If the check succeeds, the character may continue with the action. If the Concentration check fails, the action automatically fails. The check DC depends on the nature of the distraction.

Damaged during the action110 + damage dealt
Taking continuous damage during the action210 + half of continuous damage last dealt
Vigorous motion (bouncy vehicle ride, small boat in rough water, belowdecks in a storm-tossed ship, riding a horse)10
Violent motion (very rough vehicle ride, small boat in rapids, on deck of storm-tossed ship, galloping horse)15
Extraordinarily violent motion (earthquake)20
Entangled in net or snare15
Grappling or pinned20
Weather is a high wind carrying blinding rain or sleet5
Weather is wind-driven hail, dust, or debris10

1Such as an activity that requires more than a single full-round action. Also from an attack of opportunity or readied attack made in response to the action being taken (for activities requiring no more than a full-round action).

2Such as from catching on fire.

Try Again: Yes, though a success doesn't cancel the effects of a previous failure, such as the disruption of an action that was being concentrated on.

Special: By making a check against DC 15, a character can use Concentration to attempt an action defensively, so as to avoid attacks of opportunity altogether. This doesn't apply to other actions that might incur attacks of opportunity (such as moving). If the Concentration check succeeds, the character may attempt the action normally without incurring any attacks of opportunity. A successful Concentration check still doesn't allow a character to take 10 on a check when he of she is in a stressful situation; the character must roll the check as normal. If the Concentration check fails, the related action automatically fails (with any appropriate ramifications), and the action is wasted, just as if the character's concentration had been disrupted by a distraction. Since Concentration checks are called for in stressful situations, a character cannot take 10 or take 20 on such checks.

A character with the Focused feat gets a +2 bonus on all Concentration checks. The concentration skill has further uses for characters using magic or psionics.

Time: Making a Concentration check doesn't require an action; it is either a reaction (when attempted in response to a distraction) or part of another action (when attempted actively).