Stamina is required to attempt missions. Every mission you accept costs an certain amount of stamina. Most missions require five stamina, but some missions with higher rewards will cost more.

Stamina is represented by a lightning bolt icon at the top right of your screen. The number before the "/" is your current stamina. The number after the "/" is your total stamina. It's possible to go higher than your maximum if you purchase bulk stamina, but you'll only recharge up to your maximum.

When you’re out of stamina, you cannot attempt any more missions.

Stamina recovers automatically over time. You will earn 1 stamina point back every 3 minutes, until your stamina slots are full again.

If you just can’t wait (and who could blame you---this game is fun!), you can buy more stamina with Gold. Stamina can also be earned as rewards from events and leveling Up.