The OMDoc (Open Mathematical Documents) format is a content markup scheme for collections of mathematical documents, including articles, textbooks, interactive books, and courses. OMDoc also serves as the content language for agent communication of mathematical services and a mathematical software bus. This documentation describes version 1.2 of the OMDoc system, the final and mature release of OMDoc 1. The system features modularized language design, OPENMATH and MATHML for the representation of mathematical objects, and has been employed and validated in various applications. Besides a complete and rigorous specification of the OMDoc document format, this book presents an OMDoc primer with paradigmatic examples for many kinds of mathematical documents. Furthermore, various applications, projects, and tool support for OMDoc are discussed. The book will become essential reading for all working mathematicians and mathematics students aspiring to take part in the new worlds of shared mathematical knowledge.