Copeptin is a stable and sensitive surrogate marker for AVP release. It can be measured reliably within a few hours and its concentration in vivo follows that of mature AVP with respect to osmotic stimuli, hypotension or stress. Over the last years, copeptin measurement has been shown to be useful in a variety of clinical indications, which are compiled in this book. Since the availability of the copeptin assay, the number of papers on this biomarker has increased almost exponentially, and it becomes increasingly difficult to read and follow all the developments. This book will help to summarize our present knowledge of this promising biomarker, and will guide the interested researcher with those open questions that call for further studies.