This book probes into the nature of grammaticalization on the basis of an in-depth study of the process of auxiliation. Auxiliation is understood as the development of constructions into markers of tense, aspect, and modality. The book is an important contribution to the theory of grammaticalization (and language change in general), grammatical typology, and also to the study of human communication. Its originality lies on the one hand in the uncovering of new linguistic facts involving, among other things, the identification of the category of the avertive as a cross-linguistically valid grammatical distinction. On the other hand, it offers a new perspective on how grammaticalization happens in everyday linguistic communication. Particularly revealing is the study of the potential role that abductive reasoning and mismatches in discourse world knowledge between speaker and hearer play as a motivation for grammatical language change. Bringing together the explanatory potential of recent grammaticalization theory, and the insights from the latest studies of the psychology of language use, this book represents a valuable achievement in the investigation of grammatical language change and linguistic communication in general.