Abstract. Xtatic is a lightweight extension of C ♯ offering native support for statically typed XML processing. XML trees are built-in values in Xtatic, and static analysis of the trees manipulated by programs is part of the ordinary job of the typechecker. “Tree grep ” pattern matching is used to investigate and transform XML trees. Xtatic’s surface syntax and type system are tightly integrated with those of C ♯. Beneath the hood, however, an implementation of Xtatic must address a number of issues common to any language supporting a declarative style of XML processing (e.g., XQuery, XSLT, XDuce, CDuce, Xact, Xen, etc.). In particular, it must provide representations for XML tags, trees, and textual data that use memory efficiently, support efficient pattern matching, allow maximal sharing of common substructures, and permit separate compilation. We analyze these representation choices in detail and describe the solutions used by the Xtatic compiler. 1