Browsing is an interactive and exploratory process for finding information in a digital library that has advantages over search term queries in many situations. Some browsers display a concept space as a node-link graph diagram, but this can look chaotic for a graph of moderate complexity. The approach taken in this paper is to suppress some of the interrelationships (links) and order the concept space as a tree by some `natural' hierarchy. The user can then explore hidden interrelationships by dynamically interacting with the system. We demonstrate the usefulness of browsing a hierarchy via this method in a prototype called MeSHBROWSE, a system for browsing terms from the NLM Medical Subject Headings tree. It displays a node-link tree diagram of the concept space and reveals hidden interrelationships when a node is clicked on by triggering related nodes scattered about the tree to become highlighted. In this paper we describe MeSHBROWSE, discuss semantic and algorithmic issues involve...