There are many problems in identifying, searching and retrieving valuable information materials on the web. Hence, digital libraries try to find solutions to interact with each other and other information centers to make accessible the valuable information. In fact, in case of interaction among libraries, there would be the possibility of accessing valuable information globally. This paper is an attempt to identify interoperability models and also the main components important for implementing interoperability projects. A survey of performed projects in this field reveals that this type of activities started from 1998, and the number of such projects increased since 2003. Also, a preliminary study reveals that   more than 50 percent of these projects originate from US. There are, in fact, three levels of interoperability: Technical, Content, and Organizational Interoperability. Considering the fact that this research emphasizes on technical level, the findings show that most digital libraries use OAI protocol, XML, and Dublin Core metadata standard.