Databases of hierarchically annotated text occupy a central place in linguistic research and language technology development. We describe a new approach to tree query which we call "Query by Annotation". Users express a query by annotating a tree, and the annotation is compiled into an expression in a path language. The result trees are overlaid with the original query, permitting the user to see why they match. Since queries and results are annotated trees, users can easily refine and resubmit their queries. The approach to Query by Annotation is motivated and exemplified using databases of linguistic trees, or treebanks.