Discourse markers and modal particles are fuzzy linguistic categories that are difficult to describe. The contributions in this volume go beyond this statement. They discuss the intersection between modal particles and discourse markers and examine whether or not it is possible to draw a line between these two types of linguistic expressions. On the basis of new synchronic and diachronic data, from speech and writing, from European and Asian languages or cross-linguistically, the authors answer the question whether discourse markers and modal particles are distinct categories, whether they form a cline, or whether modal particles are a subcategory of discourse markers. This common question shows up throughout all chapters, which makes the book to a coherent whole. By disentangling the complexity of categorizing multifunctional expressions, this book also sheds new light on the processes of meaning extension. The traditional discourse and modal functions are complemented by interactional and textual ones.