1.Briefly on contracts
The law of contracts is concerned with the enforcement of promissory obligations. Contractual liability is usually based on consent freely given in the form of an express promise or one implied in fact from the acts of the parties. In some circumstances, however, the courts will imply a promise (often called implied in law or quasi contract) in order to avoid unjust enrichment in spite of lack of consent by the party who is bound by it.
The subject matter of contract law comprises capacity, formalities, offer and acceptance, consideration, fraud and mistake, legality, interpretation and construction, performance and conditions of performance, frustration and impossibility, discharge, rights of assignees and third party beneficiaries, and remedies. It has, to a very considerable extent, preserved its unitary quality , resisting fundamental distinctions between different classes of contracts according to either the subject of the agreement or the nature of the parties.
Accordingly, with some exceptions, its principles are applicable to agreements on such varied subjects as employment, sale of goods or land, and insurance, and to such diverse parties as individuals, business organizations, and governmental entities.
It is largely state rather than federal law, but it differs usually only in detail from one state to anther. While it is still primarily case law, an increasing number of statutes deal with particular problems. The Uniform Commercial Code, for example, contains some special provisions on the formation of contracts for the sale of goods. And by the Tucker Act of 1887, as amended, one of the most significant of the federal statutes in the field, the United States government has waived its sovereign immunity in contract actions by consenting to suit in the federal courts. Some rules laid down by statute, and by case law as well, are mandatory or compulsory and cannot be avoided by the parties, while others are implicative, interpretative, or suppletory and can be varied by agreement.


1．契約簡論
契約法所關心的是實現(xiàn)所約定的義務。通常，契約責任是以自由同意為基礎的。這種同意表現(xiàn)為當事人明示的允諾或事實上由當事人通過行為而默示允諾。但在某些情況下盡管受約束的一方并未同意，法院仍會推定允諾之存在（往往稱為由法律推定的契約即準契約），以免有人不當得利。
契約法的內容，包括能力、形式、要約與承諾、約因、欺詐與錯誤、合法與否、解釋與推定、履行及其條件、契約目的無法達到和契約無法履行、免責、受讓人及受益的第三人之權利和補救方法。契約在很大范圍內保持著統(tǒng)一性而按照協(xié)議之內容或當事人之性質排斥了不同種類契約之間的基本區(qū)別。因此，除若干例外情況外，契約法原則適用于諸如個人、企業(yè)和政府實體等不同的當事人。
契約法大多是州法不是聯(lián)邦法，但各州之間的契約法只有細節(jié)之別。契約法主要雖仍為案例法，但處理具體問題的制定法日益增多了。例如，美國《統(tǒng)一商法典》就對商品銷售合同之形成設有若干具體規(guī)定，而根據作為在這方面最重要的聯(lián)邦法之一的1887年塔克法（已經修正的），美國政府已因同意在各聯(lián)邦法院應訴而在契約訴訟中放棄了主權豁免。某些由制定法（以及案例法）規(guī)定的規(guī)則是強制的，當事人不得避免；而另一些規(guī)則則是含蓄的、解釋性的、補充性的，因而可以由協(xié)議改變之。