This report is examining theoretical issues of data streams. Data streams are a way of describing data. In contrast to relational database systems, stream data are infinite and arriving continuously. Some researchers have proclaimed streams as a new paradigm of data even though streams by other may be seen as modified relations. By examining the data models and query languages of existing stream systems, it is evaluated how streams are different from relations. It is concluded that certain properties of streams make them special. However, the differences of streams and relations are not greater than, that streams may be implemented on existing database systems.