||We present a novel, yet simple, technique for the specification of context in structured documents that we call caterpillar expressions. Although we are primarily applying this technique in the specification of context-dependent style sheets for HTML, SGML and XML documents, it can also be used for query specification for structured documents, as we shall demonstrate, and for the specification of computer program transformations. From a conceptual point of view, structured documents are trees, and one of the oldest and best-established techniques to process trees and, hence, structured documents are tree automata. We present a number of theoretical results that allow us to compare the expressive power of caterpillar expressions and caterpillar automata, their companions, to the expressive power of tree automata. In particular, we demonstrate that each caterpillar expression describes a regular tree language that is, hence, recognizable by a tree automaton. Finally, we employ caterpillar expressions for tree pattern matching. We demonstrate that caterpillar automata are able to solve tree-pattern-matching problems for some, but not all, types of tree inclusion that Kilpeläinen investigated in his PhD thesis. In simulating tree pattern matching with caterpillar automata, we reprove some of Kilpeläinen's results in a uniform framework.