Open Mathematical Documents (OMDoc) is a content markup scheme for 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 book documents OMDoc version 1.2, the final and mature release of OMDoc 1. The system has been validated in varied applications, and features modularized language design, OPENMATH and MATHML for the representation of mathematical objects.