This volume is a collection of 18 papers on the communication of certainty and uncertainty. The first part introduces recent theoretical developments and general models on the topic and its relations with modality, subjectivity, inter-subjectivity, epistemicity, evidentiality, hedging, mitigation and speech acts. In the second part, results from empirical studies in medical and supportive contexts are presented, all of which are based on a conversational analysis approach. These papers report on professional dialogues including advice giving in gynecological consultations, breaking diagnostic bad news to patients, emergency calls, addiction therapeutic community meetings and bureaucratic-institutional interactions. The final part concerns the qualitative and quantitative analysis of corpora, addressing scientific writing (both research and popular articles) and academic communication in English, German, Spanish and Romanian. The collection is addressed to scholars concerned with the topical issues from a theoretical and analytical perspective and to health professionals interested in the practical implications of communicating certainty or uncertainty.