The first millennium is a period of time that commenced on January 1, AD 1, and ended on December 31, AD 1000, of the Julian calendar.

World population, which had tripled over the preceding millennium, grew more slowly during the first millennium and may have diminished. One optimistic estimate is that the world's population rose from approximately 170 to 300 million^{[citation needed]}, but other estimates vary; one estimate suggests that the world population actually declined from 400 million people to 250 million people^{[citation needed]}.

In Europe and the Mediterranean, the first millennium was a time of great transition. The 2nd century saw the peak of the Roman Empire, followed by its gradual decline during the period of Late Antiquity, the rise of Christianity and the Great Migrations. The second half of the millennium is characterized as the Early Middle Ages in Europe, and marked by the Viking expansion in the west, the rise of the Byzantine Empire in the east, and by the Islamic conquests throughout the Near East, North Africa and the Iberian peninsula, culminating in the Islamic Golden Age (700–1200 AD).