Counterspell

Countering a spell involves casting a spell to negate the effects of a targeted spell as it is cast. The process of doing so is described below.

Counter Check:

• Countering most spells involves a counter check. The base check roll is (1d20 + 1/2 x Caster Level + 2 x Spell Level).
• The DC of the spell being countered is (11 + 1/2 x Caster Level + 2 x Spell Level).

Modifiers:

• If casting the same or inverse of the targeted spell, it is automatically countered with no counter check.
• If using the Dispel Magic spell to counter, the base check is used with a maximum Caster Level of 10 (this limit is increased to 20 with Dispel Magic, Greater).
• If a spell of relative opposite effect is used (e.g. Cone of Cold vs Fireball, Expeditious Retreat vs Slow), the DM may grant up to +6 to your check. The more closely the effect mirrors the targetted spell in both type and magnitude, the greater the bonus.
• If any other spell is used, a penalty of -2 is applied if it is of the same school as the target spell, or -4 if not.

Action

Casting a counterspell requires you to have the ability to cast the spell being used to counter. Most commonly, this is accomplished through a Ready Action. Alternatively, you could cast a spell with a cast time of an Immediate Action as your counterspell.

Special

You must know a spell is being cast in order to counter it. In conditions where a spell is being cast subtly, or it's display is otherwise obfuscated, a Perception check may be needed to notice it. Under some conditions (such as a Still Spell and Silent Spell applied to the casting), there may be no means of knowing a spell is being cast, and thus no way to counter it.
If you are countering a spell, you must first attempt to identify the spell with a Spellcraft check (DC 15 + Spell Level). If you are unsuccessful, you may attempt to counter the spell blindly. Dispel Magic and Dispel Magic, Greater function normally, but any other spell used suffers an additional -6 penalty (if the spell you use happens to be the spell targeted or it's inverse, you still automatically counter it).
Counterspells may not be countered.
Arcane and Divine spells may be used interchangeably to counter each other.

Criticals

Rolling a natural 1 or 20 on a counterspell check causes the competing magics to interact in a way that generates a Wild Surge. If you roll a natural 20 in a counterspell attempt, the targeted caster suffers the effects of a Wild Surge in addition to the outcome of the counterspell. If you roll a natural 1 in a counterspell attempt, you suffer the effects of a Wild Surge in addition to the outcome of the counterspell.

Example: A 9th level enemy Sorcerer is attempting to cast the Abjuration spell Slow on your party and you, a 7th level Wizard, have identified it being cast and wish to counter it. With a Caster Level of 9 and Spell Level of 3, The DC for this counter is 11 + 4 (1/2 Caster Level) + 6 (2 x Spell Level) = 21. If you had a Ready Action with either Haste or Slow memorized, you could counter it without a check by casting one of those spells. Barring that, the 3rd level spell Dispel Magic cast as a Ready Action would give you a check of 1d20 + 3 (1/2 Caster Level) + 6 (2 x Spell Level) = 1d20 + 9, requiring an 12 to succeed. Unfortunately you did not ready a counterspell attempt, but you do have a spell available with an Immediate Action cast time, the 4th level Illusion spell Mirror Image, Greater. As a spell of a different school with an unrelated effect, a -4 modifier is applied, giving you a check of 1d20 + 3 (1/2 Caster Level) + 8 (2x Spell Level) - 4 (different school) = 1d20 + 7, requiring a 14 to succeed.