A Multiple Listing Service (MLS)  or Multiple Listings Service) is a suite of services that (1) enables brokers to establish contractual offers of compensation (among brokers); (2) facilitates cooperation with other broker participants; (3) accumulates and disseminates information to enable appraisals; (4) is a facility for the orderly correlation and dissemination of listing information to better serve broker's clients, customers and the public. A multiple listing service's database and software is used by real estate brokers representing sellers under a listing to widely share information about properties with real estate brokers who may represent potential buyers or wish to cooperate with a seller's broker in finding a buyer for the property. The listing data stored in a multiple listing service's database is the proprietary information of the broker who has obtained a listing agreement with a property's seller.

There is no single authoritative "MLS", and no universal data format. However, there is a data standard - Real Estate Transaction Standard - that is being deployed among many MLS's in North America. The many local and private databases--some of which are controlled by single associations of realtors or groupings of associations (which represent all brokers within a given community or geographical area) or by real estate brokers--are collectively referred to as the MLS because of their data sharing or reciprocal access agreements.

Seen most widely in the US and Canada but spreading to other countries in a variety of forms, the MLS combines the listings of all available properties that are represented by brokers who are both members of that MLS system and of NAR or CREA, (the National Association of Realtors in the US.

The primary purpose of the MLS is to provide a facility to publish a "unilateral offer of compensation" by a listing broker, to other broker participants in that MLS. In other words, the commission rate that is offered by the listing broker is published within the MLS to other cooperating brokers. This offer of compensation is considered a contractual obligation, however it can be negotiated between the listing broker and the broker representing the buyer. Since the commission for a transaction as well as the property features are contained in the MLS system, it is in the best interests of the broker participants (and thereby the public) to maintain accurate and timely data.

The additional benefit of the MLS system is that an MLS subscriber may search the MLS system and retrieve information about all homes for sale by all participating brokers. MLS systems contain hundreds of fields of information about the features of a property. These fields are determined by real estate professionals who are knowledgeable and experienced in that local marketplace. Whereas public real estate websites contain only a small subset of property data.

In North America, the MLS systems are governed by private entities, and the rules are set by those entities with no state or federal oversight, beyond any individual state rules regarding real estate. MLS systems set their own rules for membership, access, and sharing of information, but are subject to nationwide rules laid down by NAR. An MLS may be owned and operated by a real estate company, a county or regional real estate board of realtors or association of realtors, or by a trade association. Membership of the MLS is generally considered to be essential to the practice of real estate brokerage.