Information Security professionals today have to be able to demonstrate their security strategies within clearly demonstrable frameworks, and show how these are driven by their organization's business priorities, derived from sound risk management assessments. This Open Enterprise Security Architecture (O-ESA) Guide provides a valuable reference resource for practising security architects and designers – explaining the key security issues, terms, principles, components, and concepts underlying security-related decisions that security architects and designers have to make. In doing so it helps in explaining their security architectures and related decision-making processes to their enterprise architecture colleagues. The description avoids excessively technical presentation of the issues and concepts, so making it also an eminently digestible reference for business managers - enabling them to appreciate, validate, and balance the security architecture viewpoints along with all the other viewpoints involved in creating a comprehensive enterprise IT architecture.