This book examines medieval notions of harmful speech conduct (1300-1550) in Western Europe as reflected in Middle Dutch textual sources. Insights are drawn from an in-depth analysis of Middle Dutch ecclesiastical, secular-ethical, and legal textual sources of harmful speech. I aim to make a contribution to research concerning harmful speech conduct in the late Middle Ages. As modern medievalists have repeatedly established, harmful speech conduct aroused considerable interest among medieval authors. Lying, boasting, flattering, railing, backbiting, grumbling, false swearing, garrulous and incendiary speech were but a few of the speech acts that provoked moral condemnation all over Western Europe from the thirteenth century onward. A tongue was able to ‘break bones’ and to inflict considerable damage on the speaker, the listeners and other relevant participants of speech situations. Harmful speech even became its own category within the ecclesiastical system of the deadly sins. The tongue was held responsible for moral pollution within the spiritual realm but was also considered in need of correction in the secular-ethical and legislative realm. Indeed, harmful speech was treated as a sin, as moral misbehaviour, and as a crime. This research has two aims, both of which are innovative. First, the subject will be systematically analysed in three different domains in order to discover an overarching discourse. No such comparative research into medieval notions of harmful speech conduct is yet available. Second, notions of modern language theory will be used to analyse the textual sources. Until now, medieval perceptions of harmful speech conduct were primarily researched by close reading or limited use of linguistic theories; my method, however, combines a number of insights by linguistic scholars and applies them in a systematic way. The research reveals the following outcomes. Based on the analysis of Middle Dutch ecclesiastical, legal, and secular-ethical textual sources, there appears to be an overlap between ...