You are allowed to respond when your opponent plays a card, uses an ability, or ends their turn. You cannot respond to your own cards, abilities, or ending of turn.

To respond, follow these steps:
1. Wait for your opponent to pay the cost for a card or ability or announce the end of their turn.
2. Announce you want to respond.
3. Pick a tactic card in your hand or an ability on a card you control in play.
4. Play the tactic card or use the ability. (see How to Play a Card or How to Use an Ability)
5. Repeat steps 2 through 4 as many times as you want.