Robert Whittaker's five-kingdom system was a standard feature of biology textbooks during the last two decades of the twentieth century. Even as its popularity began to wane at the end of the century, vestiges of Whittaker's thinking continued to be found in most textbook accounts of biodiversity. Whittaker's early thinking about kingdoms was strongly shaped by his ecological research, but later versions were also heavily influenced by concepts in cell biology. This historical episode provides insights into important intellectual, institutional, and social changes in biology after World War II. Consideration of the history of Whittaker's contributions to the classification of kingdoms also sheds light on the impact of Cold War politics on science education and educational reforms that continue to shape the presentation of biological topics in introductory textbooks today. During the late twentieth century, Robert Whittaker's five-kingdom system was a standard feature of biology textbooks, serving as an important organizing scheme for discussing biodiversity. Even as its popularity waned at the end of the century, vestiges of Whittaker's thinking continued to be found in textbooks. Beginning with the germ of an idea in 1957, Whittaker significantly revised his concept in a series of articles published during the subsequent decade. He started with a three-kingdom system that challenged the traditional plant–animal dichotomy, quickly proposed an alternative four-kingdom system, and arrived at his well-known five-kingdom system only after a decade of critical reflection. At last, Whittaker had crafted a system that biologists and educators found attractive because it seemed to capture fundamental properties of living organisms. At its roots, the five-kingdom system was an ecological idea, but Whittaker increasingly relied on cell biology—particularly, the distinction between prokaryotes and eukaryotes—as a central organizing principle for later versions of his system. Thus, the five-kingdom system reflected important intellectual developments in biology during the post–World War II era. Equally important, the success of Whittaker's system owed much to changes in the institutional structure of biology and in science education during the Cold War. Although some of Whittaker's ideas eventually fell victim to molecular systematics, cladistics, and other recent biological developments, the persistence of his system testifies to its broad appeal.