An epistle (/ɪˈpɪsəl/; from Ancient Greek ἐπιστολή (epistolḗ) 'letter') is a writing directed or sent to a person or group of people, usually an elegant and formal didactic letter. The epistle genre of letter-writing was common in ancient Egypt as part of the scribal-school writing curriculum. The letters in the New Testament from Apostles to Christians are usually referred to as epistles. Those traditionally attributed to Paul are known as Pauline epistles and the others as catholic (i.e., "general") epistles.

The ancient Egyptians wrote epistles, most often for pedagogical reasons. Egyptologist Edward Wente (1990) speculates that the Fifth-dynasty Pharaoh Djedkare Isesi—in his many letters sent to his viziers—was a pioneer in the epistolary genre. Its existence is firmly attested during the Sixth Dynasty of the Old Kingdom, and is prominently featured in the educational guide The Book of Kemit written during the Eleventh Dynasty. A standardized formulae for epistolary compositions existed by the time of the Middle Kingdom of Egypt. The epistolary formulae used in the Ramesside Period found its roots in the letters composed during the Amarna Period of the Twentieth Dynasty. Wente describes the "Satirical Letter" found on the Papyrus Anastasi I of the Nineteenth Dynasty as an epistle which was commonly copied as a writing exercise by Egyptian schoolchildren on ceramic ostraca (over eighty examples of which have been found so far by archaeologists). Epistle letters were also written to the dead, and, by the Ramesside Period, to the gods; the latter became even more widespread during the eras of Persian and Greek domination.

Epistles in prose and verse were a major genre of literature among the Greeks and particularly the Romans. The letters of Cicero are one of the most important sources on the history of the late Roman Republic and preserve features of colloquial Latin not always in evidence in his speeches and treatises. The letters of Pliny the Younger likewise are studied as both examples of Latin prose with self-conscious literary qualities and sources for historical information. Ovid produced three collections of verse epistles, composed in elegiac couplets: the Heroides, letters written in the person of legendary women to their absent lovers; and the Tristia and Ex Ponto, written in first person during the poet's exile. The epistles of Seneca, with their moral or philosophical ruminations, influenced later patristic writers.

Christian epistles, both those in the New Testament and among the early Christian Fathers, were written in accordance with the formalized, Hellenistic tradition. This reflects the amount of Hellenistic influence upon the epistle writers, especially in the case of Paul. Classicist Steve Reece has compared thousands of Greek, Roman, and Jewish letters contemporary with Paul and observes that Paul follows many of the normal epistolary conventions.

In contrast to modern letters, epistles usually named the author at the very beginning, followed by the recipient (for example, see Philippians 1:1). The scribe (or more correctly, the amanuensis) who wrote down the letter may be named at the end of the epistle (e.g., Romans 16:22). In the absence of a postal system, the courier may also be named (e.g. Ephesians 6:21–22).

After the names of the author and recipient, Pauline epistles often open with the greeting, "Grace and peace to you." "Grace" was a common Hellenistic greeting, while "peace" (shalom) was the common Jewish greeting; this reflected Paul's dual identity in Jewish faith and Hellenistic culture. There may also be a word of thanks to the audience. In secular letters, a prayer or wish for health followed.

The body begins with a brief statement introducing the main topic of the entire body.

The epistles of the New Testament canon are usually divided as follows: