Uuencoding is a form of binary-to-text encoding that originated in the Unix program uuencode, for encoding binary data for transmission over the UUCP mail system.

The name "uuencoding" is derived from "Unix-to-Unix encoding". Since UUCP converted characters between various computers' character sets, uuencode was used to convert data that should not be translated between character sets. By encoding such data into a character subset common to most character sets, the encoded form of such data files was unlikely to be "translated", thereby destroying the file. The program uudecode reverses the effect of uuencode, recreating the original binary file exactly. uuencode/decode became popular for sending binary files by e-mail and posting to usenet newsgroups, etc.