Meta-searcher, including Lycos, Yahoo and so on, is the main method for heterogeneous information search in Internet. But these mechanisms do not satisfy the demands for precision and recall of distributed information retrieval. Compatibility and interoperability among digital libraries and other information systerns is an important issue. This article explains architecture of digital libraries and models of distributed searching, specially, those models designed with metadata and Z39.50, which are the most common searching modes nowadays.