序，又称序言、前言、叙、绪、弁言、引等，是作品或书籍的一种依附性文章，主要介绍作品的内容、主旨，或者作者的创作过程，或对作品加以评论。中国最早的序，一般认为是汉朝人为《诗经》所做的《毛诗序》以及为各诗所做的小序。

序一般位于作品之前，也可位于书后，如司马迁的《太史公自序》即位于《史记》结尾，位于篇末的序为“后序”，通常称为跋或后记。由作者本人写的序为自序（Preface），作者以外的他人所写的序为他序（Foreword）。

A prologue or prolog (Greek πρόλογος prólogos, from pro, "before" and lógos, "word") is an opening to a story that establishes the setting and gives background details, often some earlier story that ties into the main one, and other miscellaneous information. The Ancient Greek prólogos included the modern meaning of prologue, but was of wider significance, more like the meaning of preface. The importance, therefore, of the prologue in Greek drama was very great; it sometimes almost took the place of a romance, to which, or to an episode in which, the play itself succeeded.