The Cambrian is the first geological period of the Paleozoic Era, lasting from 542 ± 0.3 to 488.3 ± 1.7 million years ago (mya) (ICS, 2004); it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätten. These are sites of exceptional preservation, where 'soft' parts of organisms are preserved as well as their more resistant shells. This means that our understanding of the Cambrian biology surpasses that of some later periods.

The Cambrian Period marked a profound change in life on Earth; prior to the Cambrian, living organisms on the whole were small, unicellular and simple. Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralised – hence readily fossilised – organisms became common. The rapid diversification of lifeforms in the Cambrian, known as the Cambrian explosion, produced the first representatives of many modern phyla, representing the evolutionary stems of modern groups of species, such as the arthropods. While diverse life forms prospered in the oceans, the land was comparatively barren – with nothing more complex than a microbial soil crust. Most of the continents probably resembled deserts.^{[citation needed]} Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia.The seas were relatively warm, and polar ice was absent for much of the period.