In Magic, triggered abilities are effects that occur on a conditional basis, and must be triggered by another event.

Triggered abilities can only take effect when a card is already on the battlefield, unless it explicitly says otherwise on the card. Some examples of events that can cause these abilities to go off are things like a player drawing a card, or a Creature leaving the battlefield.

Who is affected by triggered abilities?

Triggered abilities can affect you or your opponents depending on what the card states. Because triggered abilities can impact you and/or your opponent, this can make for some interesting interactions in a game.

For example, Fevered Visions states "At the beginning of each player's end step, that player draws a card. If the player is your opponent and has four or more cards in hand, Fevered Visions deals 2 damage to him or her."

There's a few things happening here:

1. Fevered Visions checks if it's a player's end step

2. When it is, that player will draw a card
3. Fevered Visions checks if the player who drew is its controller
4. If you are not the controller it counts the cards in your hand
5. If all the conditions are met, Fevered Visions will deal 2 damage to you

Can you have multiple triggers?

You can have multiple triggered abilities in a few ways.

If you have a Fathom Mage and an Angelic Chorus on the board, and play another Creature, you will gain life equal to its toughness and, Fathom Mage will get +1/+1 if it's stronger than her.

In this interaction, you actually get a third trigger: when the Mage gets a counter, you get a card.

In a 60-card format, with two copies of Angelic Chorus on the board, if you played that same Creature, you would gain life equal to its toughness twice. That amounts to one for each trigger; keep in mind that these can be countered individually.

If you're using a card like Peter Parker's Camera, you can pay mana to copy a triggered ability and will have the option to choose new targets. In practice, that looks like getting a +1/+1 from Fathom Mage, paying and tapping the Camera to copy it, and getting another +1/+1 on either the same or a different Creature.

Can I respond to a triggered ability?

You can respond to most triggered abilities. They go onto the stack when priority passes, as part of the background checklist that is happening in the game.

When conditions are met that cause an ability to trigger, or if there are multiple, the game performs state-based actions, then attempts to resolve the ability. At that time, your opponents have the opportunity to interact or attempt to counter with cards like Stifle.

Stifle is a cost Instant that counters "target activated or triggered ability," making it an inexpensive answer to many threats.

Examples of Triggered Abilities

If/When Triggers

The most common triggered abilities use words like "if..." or "when..."

These abilities can happen when a card enters or leaves the battlefield, when they attack, or when you play an Enchantment or Artifact to attach to them. You can use these effects to support your own board, like with Abzan Ascendancy, pumping your whole board when it comes out.

There are also effects like we see on Fireblade Charger, triggering when the Creatures dies and throwing some negative consequences to a Creature or player of your choice.

You can also get a boon from these effects in other ways, like allowing you to pay a lower cost for a spell if certain conditions are met, like Cobra Trap.

Whenever Triggers

"Whenever" triggers are similar but they can offer you repeated benefits. You can use triggered abilities to apply disadvantages to your opponent or enrich yourself.

For example, Coercive Impetus forces the Enchanted Creature to attack if able, causing its controller to draw a card and lose a life. On the other hand, Arbaaz Mir pings your opponents for one and gains you a life just for putting a body on the board.

At... Triggers

Triggered abilities that happen "at" certain times usually happen every turn. These are often things that hang out on the battlefield and rack up value. Agent of Masks siphons a life from every opponent and lets you gain one for each, at the beginning of every upkeep, as long as it lives.

Air Nomad Student is another good one. On your end step, if he didn't attack, he keeps growing until you need him either to block in a pinch or to come out as a heavy hitter a few turns down the line.

Final Answer

Triggered abilities occur once specific conditions are met. There are a variety of different triggered abilities that happen if you perform an action, at a certain time, or under other conditions. They can hurt your opponents, add value to your side of the board, and help you eek out a win if your wincon is sitting in the graveyard.