A bidding system in contract bridge is the set of agreements and understandings assigned to calls and sequences of calls used by a partnership, and includes a full description of the meaning of each treatment and convention. The purpose of bidding is for each partnership to ascertain which contract, whether made or defeated and whether bid by them or by their opponents, would give the partnership their best scoring result.

Each bidding system ascribes a meaning to every possible call by each member of a partnership, and presents a codified language which allows the players to exchange information about their card holdings. The vocabulary of bidding is limited to 38 different calls - 35 level/denomination bids^[1] plus pass, double and redouble. Any bid becomes a contract if followed by three successive passes, therefore every bridge bid is a potential contract.

By the rules of the game, the agreed meanings of all calls must be public and known to the opponents. In normal club or home play, the opponents are entitled, at their turn to make a call, to ask the partner of the bidder about the meaning of the call. In high-level tournaments, where screens are used, the procedure is to ask the screen-mate about their calls as well as their partner's calls. In serious online tournaments, the procedure is for the player making the call to self-alert it, but the explanation is visible only to the opponents.

Bidding systems can be classified into two broad categories: natural systems and artificial systems. In natural systems, most bids (especially in the early phase of the bidding) denote length in the suit bid. In artificial systems, the bids are more highly codified, so that for example a bid of 1♣ may not be related to a holding in the club suit.

Natural system(s) are the "lingua franca" of bridge players, with regional variations. Thus, a new partnership can agree to play a natural system and understand each other fairly well. Players sometimes alter certain aspects of a system, adding their specific agreements or preferred conventions.

Structure and meaning of opening bids are the common determining factor for system classification: in most modern natural systems, opening bids of 1♣ through 2♣ have the same or similar meaning, with level-one bids denoting length in a suit. Artificial systems typically reserve at least one one-level suit opening bid for special purposes, unrelated to the suit.

• Bridge conventions
• List of bidding systems
• Standard American
• Acol