1. A system for network content monitoring, comprising: a transport data monitor, connectable to a point in a network, for monitoring data being transported past said point, a description extractor, associated with said transport data monitor, for extracting descriptions of said data being transported, a database of at least one preobtained description of content whose movements it is desired to monitor, and a comparator for determining whether said extracted description corresponds to any of said at least one preobtained descriptions, thereby to determine whether said data being transported comprises any of said content whose movements it is desired to monitor.