MonaSearch is a new powerful query tool for linguistic treebanks. The query language of MonaSearch is monadic second-order logic, an extension of first-order logic capable of expressing probably all linguistically interesting queries. In order to process queries efficiently, they are compiled into tree automata. A treebank is queried by checking whether the automaton representing the query accepts the tree, for each tree. Experiments show that even complex queries can be executed very efficiently. The tree automaton toolkit MONA is used for the computation of the automata.