Although population estimates are available for earlier years, the first official Portuguese census was taken in 1864. It showed a population of around 4.3 million. Thereafter, the population increased slowly at rates often well under 1 % per year. Only during the 1930s and 1940s did the population increase at over 1 % per year. During the 1960s, the population actually fell by over 300,000 and in 1970 amounted to more than 8.5 million. During the early 1970s, population continued to fall or was stagnant. This demographic trend was the result of widespread emigration. Many Portuguese left their nation in these years to find employment abroad or to avoid military service in the wars Portugal was fighting in its colonies in Africa.

western Iberia has been occupied for a long time, comparatively few human remains of the Paleolithic time have been found. Neolithic and Bronze Age discoveries are more common, among them many dolmens. Some of the earliest permanent settlements were the northern castros, hill villages first built by Neolithic farmers who began clearing the forests. Incoming peoples—Phoenicians, Greeks, and Celts—intermingled with the settled inhabitants, and Celticized natives occupied the fortified castros. For 200 years these were centres of resistance to the Roman legions. consequently the Romans, Suebi, Visigoths, Moors, and Jews exerted determine on the territory. Portugal's situation at the western extremity of Europe made it a gathering place for invaders by land, and its long coastline invited settlement by seafarers.