I Doubt It! is a shedding-type card game. The object is for the player to discard all the cards from their hand.

This is done by placing one or more cards face down, presumably of a stated value, on the discard pile.

Others are allowed to challenge the truth of the player's claim about the cards being played. The game has many nicknames, the most common including BS, Cheat, and Bluff.

On each turn, the player must play the cards he has of a certain rank. The rank must be one above the rank played by the previous player (aces, then twos, threes, fours, etc to Kings, then looping back around to aces). The player selects the cards to play and places them face down in the pile in the center. If the player does not actually have the cards needed to play at least one of the correct rank, he is still required to play, and so must bluff. It is permissible to bluff when the player does have cards of the given rank, for example, by playing "two Kings" when they have only one, or playing "two Kings" and playing two of any other card.

Each play is open to challenge by any other player. The challenging player calls out “I Doubt It!” and turns over the cards that were just played to see if they matched the current rank being played. If the cards were correctly played, then the challenger must pick up the entire stack; otherwise the bluffer must pick it up. In either case, the game continues with the next player in turn playing the next rank.

The first player to get rid of all cards, and not lose a challenge on the final play, is the winner.

I Doubt It! supports solo mode (1 human player and up to 9 computer players) and multiplayer mode (up to 4 human players).