This book presents a cognitive semantic model of what the whole range of functions is that English and German discourse particles can fulfil, how these functions are related, why discourse particles fulfil just these functions and not others, and what factors condition their interpretation. Methodologically, conversation analysis and various methods from lexical semantics, such as field analysis and semantic decomposition, as well as contrastive studies are combined with the statistical analyses of large corpora, simulation experiments involving supervised learning in artificial neural networks, and the representation of the results in a computational lexicon. This methodologically interdisciplinary study thus not only presents a general model of polysemy which includes structural aspects, a conceptual background frame, and the contribution of the lexeme, it also provides full coverage for the functional readings of discourse particles and a unified definition of the word class.