This article is a bit longer than most, but since it is at the heart of the game, it's worth reading all the way through.

The result of this stacking behavior is that the spell or ability put on the stack first, resolves last. This leads many people to the mistaken assumption that when you respond to someone's spell or ability, that yours goes first. But it does not really.

This deserves repeating: responding to someone's spell or ability does NOT make everything about your spell or ability happen before theirs.

The way the stack works, players put a spell or ability on the stack by playing it. Playing involves paying costs and choosing targets (see Crystal Clear - 1A for more details). As additional spells and abilities are added to the stack, you will see that costs are paid in the order the spells and abilities are played.

Later, when the spells and abilities resolve (see Crystal Clear - 1B for more details), they will come off the stack in reverse order. Here you will see that effects occur in the reverse order.

Let's go through an example here. Bob's Prodigal Sorcerer ("{Tap}: Prodigal Sorcerer deals 1 damage to target creature or player.") looks threatening, so Carl decides to play Twiddle ("Tap or untap target artifact, creature or land.") to tap it. Bob responds by playing his Sorcerer's ability.

Showing the stack like a stack of plates looks is awkward in practice, so the examples here show spells and abilities stacking to the right. Each indentation is a new level on the stack. This allows us to show time passing as you read down the page. Here is how the stack looks over time:

A	B	C	D
A1) Bob plays Twiddle : He pays {U} and chooses Carl's Prodigal Sorcerer as the target.
	B1) Carl plays Prodigal Sorcerer 's ability: He taps the Sorcerer and chooses Bob as the target. It goes on the stack (shown indented here to mean that it is on top of the Twiddle.)
		Both players pass
	Resolve B1) Resolve the Sorcerer ability: The Sorcerer deals 1 damage to Bob.
	Both players pass
Resolve A1) Resolve the Twiddle: Bob chooses whether to tap or untap, and chooses tap. Since the Sorcerer is already tapped, this does nothing.

This is a very simple example, but it demonstrates how the Stack works.

As you can see, the costs and targeting choices happened in order, then the effects happened in reverse order. The important thing to notice here is that you can't stop someone from playing something by using a spell or ability because your effect won't happen until after they have a chance to pay that cost.

As you work to master Magic timing rules, you should practice writing out the stack using this format. Lay out the order spells and abilities are played, noting costs paid and targeting choices. Then lay out the effects of resolving spells or abilities in reverse order. You may be surprised as just how clear many complex timing questions become.

The only other important note about the stack is that it is not just added to and then emptied. Each time a spell or ability resolves and is removed from the stack, new spells and abilities can be added. Here's a slightly more complex stack example:

A	B	C	D
A1) Bob plays Lightning Bolt : He pays {R} and chooses Carl as the target. It goes on the stack.
	B1) Carl plays Healing Salve : He pays {W}, chooses to prevent damage, and chooses himself as the target. It goes on the stack (shown indented here to mean that it is on top of the Lightning Bolt.)
		C1) Bob plays Counterspell : He pays {U}{U} and chooses Healing Salve as the target.
			Both players pass
		Resolve C1) Resolve the Counterspell: Healing Salve is countered and removed from the stack.
	Healing Salve (B1) won't resolves since it is no longer on the stack
	B2) Carl plays Samite Healer 's ability: He taps the healer and chooses himself as the target.
		Both players pass
	Resolve B2) Resolve the Samite Healer ability: The next 1 damage dealt to Carl will be prevented.
	Both players pass
Resolve A1) Resolve the Lightning Bolt: Carl takes 2 damage (3 minus the one that was prevented).

Notice how spells are added and removed several times without emptying the stack?

To wrap up, there are some limitation as to what can go on the stack. Instants and abilities can always be used to start a Stack or to add to a stack. Artifact, Creature, Enchantment, and Sorcery spells (and abilities played as a sorcery) can only be played to start a stack and only during a player's Main Phase. This means that it is not possible to add a Sorcery or Creature to a stack. Playing a land does not go on the stack. It's a special action.

Once you get a feeling for the separation of playing (see Crystal Clear - 1A) and resolving (see Crystal Clear - 1B) spells and abilities, and how that looks on the Stack, you're a most of the way to understanding timing in Magic. The next topic is Priority (see Crystal Clear - 1D).

The limitation as to what can go on a stack is also straightforward. Instants and abilities can always be used to start a Stack or to add to a stack. Artifact, Creature, Enchantment, and Sorcery spells (and abilities played as a sorcery) can only be played to start a stack and only during a player's Main Phase.[408.1d]

Playing a land does not go on the stack. It's a special action. So is turning a face down permanent face up, and ending an effect that has an optional ending. [408.1i]

Mana Abilities do not go on the stack either. They are immediately resolved as soon as they are played without going on the Stack in between. [408.2e]

The Priority rules (see Crystal Clear - 1D) expand on when players (and game rules) can add and remove from the stack.

Q: So is there just one stack each step of the turn?

No. Stacks can be built up and then empty over and over without ending the step of the turn.

Q: Stacks still get deep and complex don't they?

Only rarely. Most of the time when it is your turn, it is to your benefit to keep the stacks with only one spell or ability in them. You play something, let it resolve, and then play something else, and so on.

This is because something that isn't resolved yet is vulnerable to being countered or having its target made illegal. Also, revealing your strategy one spell or ability at a time makes it more difficult for your opponent.

Q: So when is the stack important?

It is important for taking actions as a reaction to your opponent. If they play a dangerous spell, you might counterspell it. If they try to destroy one of your permanents, you might sacrifice it, use its ability, or otherwise try to get the most out of it before it is destroyed.