The volume and areal extent of Arctic sea ice is rapidly declining, and to put that decline into perspective we need to know the history of Arctic sea ice in the geologic past. Sedimentary proxy records from the Arctic Ocean floor and from the surrounding coasts can provide clues. Although incomplete, existing data outline the development of Arctic sea ice during the last several million years. Some data indicate that sea ice consistently covered at least part of the Arctic Ocean for no less than 13–14 million years, and that ice was most widespread during the last approximately 2 million years in relation with Earth’s overall cooler climate. Nevertheless, episodes of considerably reduced ice cover or even a seasonally ice-free Arctic Ocean probably punctuated even this latter period. Ice diminished episodically during warmer climate events associated with changes in Earth’s orbit on the time scale of tens of thousands of years. Ice cover in the Arctic began to diminish in the late 19th century, and this shrinkage has accelerated during the last several decades. Shrinkages that were both similarly large and rapid have not been documented during at least the last few thousand years, although the paleoclimatic record is sufficiently sparse that similar events might have been missed. Orbital changes have made ice melting less likely than during the previous millennia since the end of the last ice age, making the recent changes especially anomalous. Improved reconstructions of sea-ice history would help clarify just how anomalous these recent changes are.