Evidentiality deals with the marking of information source, that is with means that specify how we come to know what we (think to) know. For instance, such means indicate whether knowledge derives from hearsay, or whether an inference has been based on perception or on knowledge about habits. Often these indications are vague. This book focuses on sentence adverbs and so-called function words in Slavic languages. Six of them were subject of a questionnaire survey, whose discussion, preceded by general methodological background, occupies the second part of this book. The first half contains a thorough consideration of notional links between evidentiality and related domains, first of all of epistemic modality, and it discusses the intricacies of doing lexicography of evidential marking.