Resource

Most heroes have a resource that limits how frequently they can use abilities, in addition to the cooldowns for individual abilities. A hero cannot use an ability without having the required amount of his or her resource. Resources are displayed just below the heroes' health bar, both on the bottom-left of the screen by the heroes' portrait, and directly above the heroes' model.

Mana
Most heroes use Mana. It has a maximum of 500 at level 1, plus 10 per level up, to a maximum of 790 at level 30. A hero's mana restores over time based on their mana regen, and a hero cannot use an ability if they do not have enough mana to fulfill its mana cost. Mana regenerates constantly at a rate of 3 per second at level 1, plus 0.0976 per level up, to a maximum of 5.8304 per second at level 30. Heroes can also pick up Regeneration Globes that restore 7% of maximum Mana over 5 seconds.

Mana is used as a way to add another layer of depth to sustain. There is little counterplay to heroes that can poke at the enemy with no repercussions, but if they use their abilities too much they can run out of mana giving the opponent a chance to take advantage of their weakness. Sometimes it is best for heroes to hold onto their abilities for when you really need them so they don't go "out of mana" (OOM).

No resource
There are a few heroes that do not use resources at all. These heroes are generally restricted by cooldowns or other features.