Thousands of images of children being sexually abused, or posed in sexually explicit positions, are transmitted to paedophiles via telephone lines and computers.
Using anonymous e-mail addresses; routing messages through re-mailing services that strip them of identifying features, and encrypting material are all devices that make it difficult to pin down the sources of the material.
The difficulty in tracing the origins of these exchanges is enhanced by the Net's international nature. Material read in the UK can originate overseas and messages between pornography ring members can be sent via foreign service-providers.
Investigations are reliant on police forces co-operating, as well as technical know-how.
Several cases of child pornography on the Net have come to light only when computer repairers have been called in to maintain machines that had picked up images.
But police stress that it is getting easier to track illicit material as their hi-tech awareness increases, co-operation between international police forces improves and the industry shows that it is willing to help.
David Kerr, of the Internet Watch Foundation (IWF) - set up to monitor child pornography on the Net - said: "The Internet is a lot less anonymous than people often think ... it's relatively easy to track people down."
In its first year of operation, the IWF received about 2,300 reports of illegal Web sites. This year Mr Kerr expects that figure to be closer to 5,000.
Anger at the easy availability of child pornography has inspired some computer hackers to turn vigilante, either bombarding those trading child porn with junk e-mail and computer viruses, or infiltrating the system and wiping off Web sites containing illicit material.Reuse content