In this paper we describe an approach to constraint-based syntactic theories in terms of finite tree automata. The solutions to constraints expressed in weak monadic second order (MSO) logic are represented by tree automata recognizing the assignments which make the formulas true. We show that this allows an efficient representation of knowledge about the content of constraints which can be used as a practical tool for grammatical theory verification. We achieve this by using the intertranslatability of formulas of MSO logic and tree automata and the embedding of MSO logic into a constraint logic programming scheme. The usefulness of the approach is discussed with examples from the realm of Principles-and-Parameters based parsing.1 All of these are generalizations to trees of results on strings and the monadic second order theory of one successor function originally due to Büchi (1960). The applications we mention here could be adapted to strings with finite-state automata replacing tree automata. In general, all the techniques which apply to tree automata are straightforward generalizations of techniques for FSAs.2 The current version of the MONA tool works only on the MSO logic of strings. There is work in progress at the University of Aarhus to extend MONA to "MONA++", for trees (Biehl et al. 1996).