对价是英美合同法的重要概念，而这两个为有关对价产生的重要理论。对价的引入是基于以下的原因：按照传统的观点，合同是一项或一组这样的允诺〔promise〕：它或它们一旦被违反，法律就会给予救济。而要使允诺成为一项法律能为之提供救济的允诺，即成为有法律约束力的合同，则受允诺人〔promisee〕必须向允诺人〔promisor〕提供某种与该诺言相对应的回报，这种回报就被认为是对价。

“benefit detriment theory”获益-受损公式理论。即如果允诺人从交易中获益，那么这种获益就是其作出允诺的充分对价；而另一方面，如果受诺人遭受了某种损害，那么这种损害也足以证明对方曾经作出过某种允诺。The first is "benefit-detriment theory," in which a contract must be either to the benefit of the promisor or to the detriment of the promisee to constitute consideration (though detriment to the promisee is the essential and invariable test of the existence of a consideration rather than whether it can be constituted by benefit to the promisor。

“bargain theory”变换理论。即：对价的本质在于它是作为允诺的动机或诱因而提出和接受的；反言之，允诺之做出亦是对价之给付的诱因。整件事的根本就在于对价与允诺之间的互惠引诱关系。该理论由于与商品经济的飞速发展极为适应，因此很快成为美国法中对价制度的新正统。The second is "bargain theory," in which the parties subjectively view the contract to be the product of an exchange or bargain. Bargain theory has largely replaced benefit-detriment theory in modern contract theory, but judges often cite both and may use both models in their decisions.

从“benefit detriment theory获益-受损公式理论”到“bargain theory变换理论”的转换主要是为了避免有关对价是否充分的质疑。比如说，如果某一因急于将某辆车脱手而承诺将这辆车以1美元的价格卖给你，则1美元构成充分对价。但是，如果在你的生日当天，你的朋友写下“我愿意把我的车以1美元为对价卖给你”，则这一对价则不能视为充分对价。

以下为相关双语示例，供参考：

Bargain theory has largely replaced benefit-detriment theory in modern contract theory, but judges often cite both and may use both models in their decisions. These theories usually overlap; in standard contracts, such as a contract to buy a car, there will be both an objective benefit and detriment. However, there are certain contracts which satisfy one but not the other.

变换理论在很大程度上取代了现代合同理论中的获益-受损公式理论，但法官有时会同时援引这两种理论，并可能在其判决中运用这两种模式。这些理论通常是有重叠之处的；在标准合同（如买车合同）中，会存在客观的利与损。然而，也有一些合同只符合其中一种理论。