In many recent applications, data may take the form of continuous data streams, rather than finite stored data sets. Several aspects of data management need to be reconsidered in the presence of data streams, offering a new research direction for the database community. Consequently, research has moved from the traditional database management system (DBMS) to the data stream management system (DSMS) which is powered by continuous queries. In this paper, continuous query (CQ) is introduced to motivate our discussion of its application areas.