Concentration Skill

Concentration (Con)
You are particularly good at focusing your mind.

Check
You must make a Concentration check whenever you might potentially be distracted (by taking damage, by harsh weather, and so on) while engaged in some action that requires your full attention. Such actions include casting a spell, manifesting a power, concentrating on an active spell or power, or directing a spell or power.

If the Concentration check succeeds, you may continue with the action as normal. If the check fails, the action automatically fails and is wasted. If you were in the process of casting a spell, the spell is lost.

Epic Check
You can cast spells with somatic components even while grappled.

Action
Making a Concentration check doesn’t take an action.

Try Again
Yes, though a success doesn’t cancel the effect of a previous failure, such as the loss of a spell you were casting.

Synergy
If you have 5 or more ranks in Concentration, you get a +2 bonus on Autohypnosis checks.