There are occasional studies of the interlingual translation of verse into verse and into prose, but versification of a prose “original” within the same natural language is very seldom, if ever, considered as translation, even though it fulfils the criteria of conversion, paraphrase, interpretation, and communicative mediation for a different readership that are characteristic of the translational process. Prosification began at least in the Middle Ages, and it has acquired an irrepressible momentum in Western literatures since the eighteenth century with the emergence of the modern notion of literature and the ever-increasing proportion of narrative fiction in the corpus. Nevertheless, it is largely ignored by literary historians and theorists, except in the case of the “prose poem.” After establishing essential working definitions of the notions involved, and evoking some historical examples of interlingual prosification and versification (particularly French translations of Paradise Lost), this article proposes to develop a systematic approach to intralingual versification and prosification as translational phenomena that display a similar productivity and betray the same aporetic limits as interlingual translation. They often reveal deep trends in the concepts of language, form, and representation that prevail in a particular literary culture.