Ringworld is Niven's most highly acclaimed book and also one of his most popular. The story has the simple quest plot structure that is the hallmark of much great science fiction and fantasy. Louis Wu, a two-hundred-year-old explorer, Teela Brown, a twenty-year-old, very lucky young woman, and Speaker-to-Animals, an ambassador to Earth from the catlike Kzinti, are recruited by Nessus, a centaurlike puppeteer, to explore the Ringworld, an enormous metal strip circling a star at ninety-three million miles. The Ringworld is one of the most bizarre yet logically designed and fully detailed worlds in science fiction. Since it is manmade it is also a remarkable feat of engineering. Part of the novel's appeal is the sense of wonder which such an object inspires in the reader. Traditional hard science fiction has always tried to instill this sense of wonder into its readers through the marvels of the universe or through those of technology and the Ringworld works on both levels.

During the late 1960s, the idea of improvement through science and space expansion, a central theme in hard science fiction, was challenged by growing disturbances over pollution, overpopulation, and standardization brought about by technology. This challenge shook the old confidence of American society in science and industry. Niven, however, is a defender of the traditional values of hard science fiction. Ringworld was apparently designed by its builders to provide a nearly limitless environment for a technologically advanced race with a growing population but without space travel. However one unforeseen malfunction had caused the collapse of the Ringworld civilization. The natives of the Ringworld now believe their world to be a natural phenomenon built by gods. The four explorers even masquerade as gods themselves for a time to impress the natives. Yet Niven reveals that the attempt to create a perfectly planned world is a way of playing god and is doomed to failure, that no one can set the limits of growth. Niven reaffirms the hard science fiction belief in the value of space exploration, of looking for new worlds rather than "perfecting" old ones. The travelers escape the Ringworld because Louis Wu has the imagination and flexibility gained from long years of exploring to analyze situations from different perspectives. These qualities can only come from an expanding frontier of human experience. The ability to encounter the unknown is a more important survival value than a planned society. The 1960s were a period when scientific progress came into disfavor in many quarters, but also when men began to realize the science fiction writers' dream of space travel. Ringworld's popularity is a sign that that dream can still inspire.