In commercial and consumer transactions, a warranty is an obligation that an article or service sold is as factually stated or legally implied by the seller, and that often provides for a specific remedy such as repair or replacement in the event the article or service fails to meet the warranty. A breach of warranty occurs when the promise is broken, i.e., a product is defective or not as should be expected by a reasonable buyer.

In business and legal transactions, a warranty is an assurance by one party to the other party that certain facts or conditions are true or will happen; the other party is permitted to rely on that assurance and seek some type of remedy if it is not true or followed.

In real estate transactions, warranty deed is a promise that the buyer's title to a parcel of land will be defended.

A warranty may be express or implied.

Express warranty

An express warranty is typically a guarantee from the seller of a product that specifies the extent to which the quality or performance of the product is assured and states the conditions under which the product can be returned, replaced, or repaired. It is often given in the form of a specific, written "Warranty" document. However, a warranty may also arise by operation of law based upon the seller's description of the goods, and perhaps their source and quality, and any material deviation from that specification would violate the guarantee. For example, an advertisement describing a product is often full of express warranties; the product must substantially conform to what is advertised. Many advertisers insert disclaimers for this purpose (e.g., "actual color/mileage/results may vary", or "not shown actual size"). Commonly, written warranties will assure the buyer that an article is of good quality and against defects in "materials and workmanship." A warranty may also apply to services that are sold. For example, an automobile repair shop may guarantee its repair for a period of 90 days.

An express warranty can be made orally, in writing and without the intent of the seller to actually create the warranty. In the United States, a seller is allowed to assert statements of opinion of value, known as puffery, that the buyer cannot justly rely on as part of the basis for the bargain. For instance, "This hunting knife is the best knife in the world" is mere puffery, whereas a statement such as "This hunting knife will never need to be sharpened" can be construed to be an express warranty as long as the knife is only used for its intended purpose. In certain other countries (e.g. the UK, Canada, and Taiwan), consumer protection laws exist to prevent advertisers making untrue or unprovable statements.