Interrupt

To interrupt an action in Guild Wars is to prevent it from being completed. Most skills have an activation time during which the character is preparing the skill. If a character is interrupted during skill activation, the skill is interrupted. This has four results:
 * 1) The interrupted skill has no effect.
 * 2) The skill's cost (energy or adrenaline) is forfeited.
 * 3) The skill cannot be used again until the normal recharge time for that skill passes.
 * 4) The interrupted actor does not have to go through aftercast for the interrupted skill.

There are various ways to stop enemy skills from activating:
 * Actual interrupts (skills with the word(s) "interrupt" in their description, or being attacked while dazed) will immediately interrupt the target (unless protected by Mantra of Concentration or such).
 * Some skills (including ones that disable skills) cause the target's skill to fail after the activation time has passed (e.g. Guilt, Blackout, mid-casting Spell Breaker). Mantra of Concentration and similar do not help against them.
 * Knocking down enemies interrupts them at the same time. Skills that prevent interrupts don't work against knockdown.

Tips on interrupts

 * To effectively interrupt, watch the Skill Monitor. which is by default under the enemy's health bar in the center of the top of the screen. When the enemy starts using a skill, the skill's activation bar appears under the bar. If it's a skill you want to interrupt (and can interrupt), do so immediately.
 * Sometimes, if an opponent is casting a spell or using a skill that takes a very long time to prepare (like Resurrect or the like), it may be in your interest to wait until the last second to interrupt the player. In this case, he/she will have lost not just energy, but also the entire casting time of the spell.  Keep in mind that some skills have no recharge, so interrupting them immediately only allows the player to begin recasting the spell right away.
 * It is best to study (and perhaps play as) other professions to truly understand what is the best way to interrupt each class.
 * Interruption is not just for spells. Other actions (including attacks and attack skills and signets) can be interrupted. Attacks have an activation time (see the specific weapon's page) and take time to execute. Any such action or skill can be interrupted, though there are more skills that interrupt spells than attacks.
 * Some skills (such as stances and shouts) don't have an activation time. These skills can't be interrupted.

"Easily interrupted"
Some skills are easily interrupted (including all traps). Characters activating these skills are interrupted if any attack hits, even without the use of interruption skills. All of a character's spells are easily interrupted while that character is dazed.

Related articles

 * Disable skills quick reference
 * Interrupt skills quick reference
 * Skill failure
 * Fail skills quick reference
 * See also: dazed and knockdown

These skills are easily interrupted:
 * Crude Swing
 * Barbed Trap
 * Dust Trap
 * Flame Trap
 * Healing Spring
 * Precision Shot
 * Spike Trap
 * Viper's Nest
 * Snare
 * Smoke Trap
 * Tripwire
 * Ethereal Light

These skills prevent interruption (but knockdown, disabling, and causing to fail will interrupt despite any of these):
 * Mantra of Concentration prevents one interruption.
 * Mantra of Resolve prevents interruptions at the cost of energy each time.
 * Persistence of Memory makes your interrupted spells recharge instantly.
 * Glyph of Concentration prevents interruption of the next two spells cast and ignores the effects of Dazed.
 * Trapper's Focus makes traps 'not easily interruptible' for a duration.
 * Tranquil Was Tanasen is an Item Spell that prevents interruption.
 * Song of Concentration prevents interruption of the next skill used by ally in earshot.
 * Pious Concentration expends one enchantment per interrupt prevented.