This thesis describes the design of a full-¿edged XML storage and query architecture, which represents the core of the Open Source database system BaseX. In contrast to numerous other works on XML processing, which either focus on theoretical aspects or practical implementation details, we have tried to bring the two worlds together: well-established and novel concepts from database technology and compiler construction are consolidated to a powerful and extensible software architecture that is supposed to both withstand the demands of complex real-life applications and comply with all the intricacies of the W3C Recommendations.