XQuery 1.0 and its sister language XPath 2.0 have set a fire underneath database vendors and researchers alike. More than thirty commercial and research XQuery implementations are listed on the XML Query working group home page. Galax [FS] is an open-source, general-purpose XQuery engine, designed to be complete, efficient, and extensible. During Galax's development, we have focused on each of these three requirements in turn, while never losing sight of the other two. Our success or failure in satisfying these requirements depends entirely on the design and implementation of Galax's architecture. We describe Galax's architecture in detail and identify several key principles that guide our decisions on Galax's design and implementation.