Sharks and their biological cousins, the rays, are among the highest-profile denizens of the deep. But sharks are not the solitary killing machines that popular movies and the press might have us believe. In their marine environment, sharks coexist with numerous other species – many of whom flock to be near the sharks, rather than running from them in fear. These excerpts from the NATURE episode “The Secret World of Sharks and Rays” examine the interrelationships between sharks and other marine species. In many of these cases, the interaction between two different species mutually benefits each species. But humans, too, have become an increasingly important player in the lives of sharks – and as they are increasingly hunted for their fins, sharks are actually becoming more endangered than they are dangerous. The impact on the marine ecosystem that would result from the disappearance of sharks would be devastating, but there is still time to save these magnificent creatures, and the ecosystems that depend on their existence.