Responsa (Latin: plural of responsum, "answers") comprise a body of written decisions and rulings given by legal scholars in response to questions addressed to them. In the modern era, the term is used to describe decisions and rulings made by scholars in historic religious law.
Roman law recognised responsa prudentium, i.e. the responses and thoughts of jurists, as one of the sources of "Ius scriptum" (written law), along with laws originating from magistrates, from the Senate, or from the emperor.
A particularly well-known and highly influential example of such responsa was the Digesta (or Digests), in 90 books, principal work of the prominent Second Century jurist Salvius Julianus. This was a systematic treatise on civil and praetorian law, cited by many later Roman legal writers, which has been described as "A comprehensive collection of responsa on real and hypothetical cases; in general, it followed the edictal system... With Iulianus, the Roman jurisprudence reached its apogee." 
In the Roman Catholic Church, a responsum is an answer given by the Congregation for the Doctrine of the Faith on certain matters pertaining to faith and morals. The Holy Office is the sole and exclusive doctrinal organization which has the right to give responsa. Recent doctrinal documents which contain relevant responsa are Commentary on Responsa ad quaestiones, Responsum ad Dubium Concerning the Teaching Contained in Ordinatio Sacerdotalis and Responses to Certain Questions of the United States Conference of Catholic Bishops Concerning Artificial Nutrition and Hydration.
In rabbinic literature, the Responsa are known as She'elot u-Teshuvot (Hebrew: ? "questions and answers"), and comprise the body of written decisions and rulings given by poskim ("deciders of Jewish law"). A modern term, used mainly for questions on the internet, is "Ask the rabbi".
Judaism's responsa constitute a special class of rabbinic literature, to be distinguished from the commentaries (meforshim)--devoted to the exegesis of the Hebrew Bible, the Mishnah, the Talmud--and from the codes of law which delineate the rules for ordinary incidents of life.
The responsa literature covers a period of 1,700 years--the mode, style, and subject matter have changed as a function of the travels of the Jewish people and of the development of other halakhic literature, particularly the codes.
Responsa play a particularly important role in Jewish law. The questions forwarded are usually practical, and often concerned with new contingencies for which no provision has been made in the codes of law, and the responsa thus supplement the codes. They, therefore, function as a source of law, almost as legal precedent, in that they are consulted by later decisors (poskim) in their rulings; they are also, in turn, incorporated into subsequent codes.
In addition to requests for halakhic rulings, many of the questions addressed were theoretical in character, particularly among the earlier responsa. The responsa thus contain rulings on ethics, business ethics, the philosophy of religion, astronomy, mathematics, history, geography, as well as interpretations of passages in the Bible, the Mishnah, the Talmud, and the Midrash. Thus, while early Jewish literature has few historical works, many notes on the history of Judaism have been introduced into the responsa.
Responsa thus contain valuable information about the culture of the Jews and the people among whom they lived. Information may also be gleaned about the moral and social relations of the times, occupations, the household, customs, expressions of joy and of sorrow, and recreations, and even games. Older responsa are also important for readings and emendations of the Mishnah and the Talmud.
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A similar use of responsa (here called fatw?) is found in Islam. The mufti is a member of the Islamic scholarly class (ulam?) who form the Muslim religious establishment. In Islam, the term muft? is largely restricted to Sunnism, and has both a formal and informal use, the former for state-appointed officials who gave rulings on matters concerning the state or the public, the latter for individuals who respond to their followers or to others.
In Shiaism, fatwa is also used. There personal devotion to specific clergy is mandatory for believers. High-ranking members of the ulama class achieve the status of marja? al-taql?d (pl. mar?ji?), that is, "the point to which imitation returns": in other words, they pronounce on religious matters, especially legal ones, and the rest of mankind are muqallid or imitators, who do nothing without the mandate of their specific marja?. There are very few mar?ji? at any time, though on a number of occasions since the 19th century, the title has come to rest on a single individual for the entire Shia world. There are larger numbers of Shia clergy with the rank of mujtahid, who are empowered to give independent opinions on religious matters. Traditionally, as in Judaism, the answers of mar?ji? and mujtahid are collected in a compilation called Ris?la-yi su'?l va jav?b (Farsi) or "Epistle of Questions and Answers."
There is generally greater latitude for Shia ulam?, insofar as the principle of independent reasoning (ijtih?d, from the same root as mujtahid) in matters of religious law remains valid in Shi'i jurisprudence, whereas it is deemed[by whom?] to have ended in Sunnism as far back as the 10th century.