The purpose of paragraphing is to make text understandable and easy to read, and to help you tell your story effectively. Paragraphing is difficult because the purpose of the documents we produce and their readership are diverse. To make matters worse, little guidance is given in school and during higher education. Paragraphing is not governed by standard rules; some conventions apply but often are – or have to be – ignored in scientific and medical texts. This is the first of two articles on paragraphing and deals with basic issues that face medical writers and editors. The second article will look at developing paragraphs from ideas when you plan a document.