The First Men are the original human inhabitants of Westeros. They ruled the continent for thousands of years before the Andals invaded from the eastern continent of Essos. The Andals overran most of southern Westeros, but failed to take the North. While the blood of the First Men and the Andals has heavily intermingled over the centuries (through marriage or otherwise), the inhabitants of the North have the greatest amount of First Men blood in their veins and still maintain many of their traditions.