Sexual predators have grown increasingly adept at using the internet to share and view child sexual abuse photos and videos. Some have computer-programming skills and have deployed sophisticated defenses against efforts to take them down.

But the predators don’t have free rein on the internet, thanks to nearly four dozen child protection hotlines around the world, which act as a first line of defense against the explosion of imagery.

[Read The New York Times’s investigation into the global fight against online child sexual abuse.]

The hotlines play a central role in getting tech companies, websites and others to address the content. When the hotlines become aware of an illegal image, they issue a notice to have it removed. They may also notify law enforcement officials, who can launch a criminal investigation and try to rescue the abused child.

But to keep up with the predators, the hotlines must constantly find ways to outsmart them. The problem is that no solution lasts very long, because the predators also use technology to find ways to hide.