- Sometimes, you’ll find yourself in a position where you can easily play your whole hand while your opponent is still holding on to close to the maximum of 10 cards. Unless you can win the game this turn, my advice would be: Don’t. Why? Card advantage. - The Innkeeper[1]
Card advantage is a term used to describe which player has access to more cards in a given situation. In Hearthstone this is often an indicator of who is ahead in the game.
Terminology[]
Card advantage is an equation which takes both sides into account. 
Arcane Intellect draws 2 cards, but also loses 1 card (Arcane Intellect itself). Changes in card advantage are typically expressed as "X for Y" where X is the number of cards the player gains and Y the number they lose. The above example of Arcane Intellect would result in an "2 for 1" which indicates a positive outcome for the player since the gain is higher than the loss.
Card advantage is ultimately an expression of the difference between the rate at which a player gains cards, and the rate at which they lose cards, in comparison to that of the opponent. If the player is gaining more cards each round than they are losing, they will be accumulating card advantage - provided the player does not exceed the 10 card hand limit, or exhaust his deck. If the player is slowly seeing a net loss of cards, but the opponent is losing them faster, this is another form of card advantage.
Note that card advantage can involve both cards in the player's hand, and cards placed on the battlefield as minions. If a player can play a single spell card to destroy several of their opponent's minions, they will have gained card advantage. Similarly, if a player is forced to use several spells to destroy a single minion, the minion can be considered to have generated card advantage for its owner. While a spell card is generally considered to have been used or 'lost' once it is played, a minion card is generally only considered lost once it has been removed from the board.
Strategy[]
Controlling the rate at which the player and their opponent gains and loses cards is key to attaining and maintaining card advantage. There are several methods for doing so:
- Card draw - This is the simplest way of gaining card advantage. These cards generate card advantage by simply drawing you more cards than you lose. This may be by drawing several cards (e.g. Arcane Intellect, Nourish, Sprint) or by providing you with a card in addition to a minion (e.g.
Novice Engineer). The warlock hero power Life Tap creates card advantage by drawing one card, in exchange only for Health and mana. Card draw can also come from minions through specific events, such as Northshire Cleric and Starving Buzzard, offering potentially unlimited card draw in addition to the minion provided.
- Area of effect (AoE) - These abilities and spells may clear the battlefield of multiple minions. Every class in Hearthstone has one or more cards that are capable of doing this, and several neutral minions can do the same. AoE generates card advantage by trading the card itself (a loss of 1) for the death of several opponent minions (a gain of 2 or more). Example: Player A has 3 minions in play. 2 of them were played from the hand, 1 is a Silver Hand Recruit summoned with Reinforce. Player B now plays Starfall, destroying all minions of Player A. This results in a 2-for-1 trade for player B and a 1-for-2 trade for Player A. Although Player B kills 3 minions, it is not an 3-for-1 trade since one minion was not created with a card, and therefore does not represent a card lost by Player A. Area of effect cards are occasionally called "sweepers", a term borrowed from other card games.
- Favorable combat - The most common source of card advantage is favorable combat. Whenever 2 or more minions trade with 1 minion, card advantage for the controller of the single minion is created (with the exception of minions that did not come from a card). Establishing card advantage through favorable fighting is the common way of winning games in the Arena, since more refined strategies are less likely to be available.
- Economy - If a player is consistently losing fewer cards each round than their opponent, they will be gaining card advantage. One way to improve card economy is to make use of hero powers. For example, by using Fireblast to kill a Young Priestess, a mage can destroy one of their opponent's cards without using one of their own. Similarly, the shaman and paladin hero powers Reinforce and Totemic Call can be used as substitute for low value minion cards, allowing the player to populate the board without using any cards. While hero powers generally offer an inefficient mana economy, their regular use can allow the player to accumulate significant card advantage while still responding to events on the board.
Virtual card advantage[]
The concept of virtual card advantage describes the lack of usefulness of cards in certain situations. Whenever a card is not usable, for example because there are no valid targets for the card, that player's hand is virtually down by one card. For example, 
Bewitch may generate Totemic Might when the player has no totems or ability to generate totems.
The same goes for cards with expensive casting cost. As long as a player does not have the option to play a card, it is useless.