It is very important to realize an adaptive network that can dynamically adapt itself to users' needs and data traffic. We are developing the Active Network (AN) environment as a basic mechanism to realize such a network. AN is a network technology that enables us to introduce new services easily by attaching programs to the packets and evaluating the programs at each router. After introducing our AN environment, this article describes the active router, the most important part of the AN. In addition, a streaming multicast application that uses the AN is presented.