The report describes active networks and how they can be used. Current research results are briefly described and the report also contains an analysis and evaluation of the security in some implementations of active networks. The functionality of an active network can be changed by sending program code (mobile code) between the nodes in the network. This makes it possible to treat the network as a programmable shared computer. Even today you can see noticeable trends that the networks become more active. Examples are firewalls and webproxies, which can be configured and controlled by fast loading of new program code. The disadvantage is increased security risks when using this form of mobile program code.