Quid pro quo ('what for what' in Latin) is a Latin phrase used in English to mean an exchange of goods or services, in which one transfer is contingent upon the other; "a favor for a favor". Phrases with similar meanings include: "give and take", "tit for tat", "you scratch my back, and I'll scratch yours", and "one hand washes the other". Other languages use other phrases for the same purpose.
The Latin phrase quid pro quo originally implied that something had been substituted, as in this instead of that. Early usage by English speakers followed the original Latin meaning, with occurrences in the 1530s where the term referred to substituting one medicine for another, whether unintentionally or fraudulently. By the end of the same century, quid pro quo evolved into a more current use to describe equivalent exchanges.
In 1654, the expression quid pro quo was used to generally refer to something done for personal gain or with the expectation of reciprocity in the text The Reign of King Charles: An History Disposed into Annalls, with a somewhat positive connotation. It refers to the covenant with Christ as something "that prove not a nudum pactum, a naked contract, without quid pro quo." Believers in Christ have to do their part in return, namely "foresake the devil and all his works". 
Quid pro quo would go on to be used, by English speakers in legal and diplomatic contexts, as an exchange of equally valued goods or services and continues to be today.
The Latin phrase corresponding to the usage of quid pro quo in English is do ut des (Latin for "I give, so that you may give"). Other languages continue to use do ut des for this purpose, while quid pro quo (or its equivalent qui pro quo, as widely used in Italian, French and Spanish) still keeps its original meaning of something being unwillingly mistaken, or erroneously told or understood, instead of something else.
In common law, quid pro quo indicates that an item or a service has been traded in return for something of value, usually when the propriety or equity of the transaction is in question. A contract must involve consideration: that is, the exchange of something of value for something else of value. For example, when buying an item of clothing or a gallon of milk, a pre-determined amount of money is exchanged for the product the customer is purchasing; therefore, they have received something but have given up something of equal value in return.
In the United Kingdom, the one-sidedness of a contract is covered by the Unfair Contract Terms Act 1977 and various revisions and amendments to it; a clause can be held void or the entire contract void if it is deemed unfair (that is to say, one-sided and not a quid pro quo); however this is a civil law and not a common law matter.
Political donors must be resident in the UK. There are fixed limits to how much they may donate (£5000 in any single donation), and it must be recorded in the House of Commons Register of Members' Interests or at the House of Commons Library; the quid pro quo is strictly not allowed, that a donor can by his donation have some personal gain. This is overseen by the Parliamentary Commissioner for Standards. There are also prohibitions on donations being given in the six weeks before the election for which it is being campaigned. It is also illegal for donors to support party political broadcasts, which are tightly regulated, free to air, and scheduled and allotted to the various parties according to a formula agreed by Parliament and enacted with the Communications Act 2003.
In the United States, if the exchange appears excessively one sided, courts in some jurisdictions may question whether a quid pro quo did actually exist and the contract may be held void. In cases of "Quid Pro Quo" business contracts, the term takes on a negative connotation because major corporations may cross ethical boundaries in order to enter into these very valuable, mutually beneficial, agreements with other major big businesses. In these deals, large sums of money are often at play and can consequently lead to promises of exclusive partnerships indefinitely or promises of distortion of economic reports, for example.
In the U.S., lobbyists are legally entitled to support candidates that hold positions with which the donors agree, or which will benefit the donors. Such conduct becomes bribery only when there is an identifiable exchange between the contribution and official acts, previous or subsequent, and the term quid pro quo denotes such an exchange.
In United States labor law, workplace sexual harassment can take two forms; either "Quid pro quo" harassment or hostile work environment harassment. "Quid pro quo" harassment takes place when a supervisor requires sex, sexual favors, or sexual contact from an employee/job candidate as a condition of their employment. Only supervisors who have the authority to make tangible employment actions (i.e. hire, fire, promote, etc.), can commit "Quid pro quo" harassment. The supervising harasser must have "immediate (or successively higher) authority over the employee." The power dynamic between a supervisor and subordinate/job candidate is such that a supervisor could use their position of authority to extract sexual relations based on the subordinate/job candidate's need for employment. Co-workers and non-decision making supervisors cannot engage in "Quid pro quo" harassment with other employees, but an employer could potentially be liable for the behavior of these employees under a hostile work environment claim. The harassing employee's status as a supervisor is significant because if the individual is found to be a supervisor then the employing company can be held vicariously liable for the actions of that supervisor. Under Agency law, the employer is held responsible for the actions of the supervisor because they were in a position of power within the company at the time of the harassment.
To establish a prima facie case of "Quid pro quo" harassment, the plaintiff must prove that they were subjected to "unwelcome sexual conduct", that submission to such conduct was explicitly or implicitly a term of their employment, and submission to or rejection of this conduct was used as a basis for an employment decision, as follows:
Once the plaintiff has established these three factors, the employer can not assert an affirmative defense (such as the employer had a sexual harassment policy in place to prevent and properly respond to issues of sexual harassment), but can only dispute whether the unwelcome conduct did not in fact take place, the employee was not a supervisor, and that there was no tangible employment action involved.
Although these terms are popular among lawyers and scholars, neither "hostile work environment" nor "quid pro quo" are found in Title VII of the Civil Rights Act of 1964, which prohibits employers from discriminating on the basis of race, sex, color, national origin, and religion. The Supreme Court noted in Burlington Industries, Inc. v. Ellerth that these terms are useful in differentiating between cases where threats of harassment are "carried out and those where they are not or absent altogether," but otherwise these terms serve a limited purpose. Therefore, sexual harassment can take place by a supervisor, and an employer can be potentially liable, even if that supervisor's behavior does not fall within the criteria of a "Quid pro quo" harassment claim.
Quid pro quo was frequently mentioned during the first impeachment inquiry into U.S. president Donald Trump, in reference to the charge that his request for an investigation of Hunter Biden was a precondition for the delivery of congressionally authorized military aid during a call with Ukrainian president Volodymyr Zelensky.
For languages that come from Latin such as Italian, Portuguese, Spanish and French, Quid pro quo is used to define a misunderstanding or blunder made by the substituting of one thing for another. The Oxford English Dictionary describes this alternative definition in English as "now rare". The Vocabolario Treccani (an authoritative dictionary published by the Encyclopedia Treccani), under the entry "qui pro quo", states that the latter expression probably derives from the Latin used in late medieval pharmaceutical compilations. This can be clearly seen from the work appearing precisely under this title, "Tractatus quid pro quo," (Treatise on what substitutes for what) in the medical collection headed up by Mesue cum expositione Mondini super Canones universales... (Venice: per Joannem & Gregorium de gregorijs fratres, 1497), folios 334r-335r. Some examples of what could be used in place of what in this list are: Pro uva passa dactili ('in place of raisins, [use] dates'); Pro mirto sumac ('in place of myrtle, [use] sumac'); Pro fenugreco semen lini ('in place of fenugreek, [use] flaxseed'), etc. This list was an essential resource in the medieval apothecary, especially for occasions when certain essential medicinal substances were not available.
Quid is slang for pounds, the British currency, originating on this expression as in: if you want the quo you'll need to give them some quid, which explains the plural without s, as in I gave them five hundred quid.