The Cambrian explosion is an excellent example of a grand idea that has been tempered by the steady collection of data to test hypotheses. Historically, the idea of an “explosion” developed from an apparent lack of bilaterian animal fossils before a certain point in the fossil record, in contrast with a great diversity of life that seemed to appear in the Cambrian period. DNA molecular clock estimates contradict this story, however, with most dates for the divergence of major phyla predating the Cambrian by 100 million to 400 million years. The contradiction might be rectified by corrections to the clock or by discoveries of Precambrian bilaterian fossils. Although many candidates exist, no single environmental or biological explanation for the Cambrian explosion satisfactorily explains the apparent sudden appearance of much of the diversity of bilaterian animal life. Scientists' understanding of this phenomenon has been greatly amplified in recent years by better geological dating and environmental characterization, new fossil discoveries, and by a great expansion of our knowledge of developmental mechanisms and their evolutionary meaning.