“Post-editing” is the term used to describe the activity of revising a text that has been translated automatically by a Machine Translation (MT) system. A basic description of the activity is that the editor, who is typically a trained translator, compares the source text with the “raw” translation produced by the MT system, identifies any errors, including omissions, unwanted additions, grammatical and stylistic problems and fixes them by revising the translation. While MT has been in development for very many years, it really only became sophisticated enough in the 1980s to warrant application in real-world translation settings, and this is the era in which we start to hear about post-editing in practice (see, for example, Vasconcellos 1985, Wagner 1985). At that time, the most common process involved sending text to the MT system and then editing it post-MT, hence the affix “post” in post-editing. At the time of writing, new ways of interacting with MT systems have been developed, which calls into question the appropriateness of the term (see Section 2.4 for further discussion). Nonetheless, the term is still very common and will probably continue to be so for some time to come. Post-editing is basically a form of translation revision, except that the post-editor is revising translation produced automatically by an MT system, rather than translation produced by a (human) translator. The task differs from the traditional notion of revision in that the types of errors generated by an MT system will differ from those created by a translator.