At the beginning of the XXI century, Portugal remains a country with low fertility, increasing life expectancy and decreasing net migration. In 2007, the natural increase was negative, a situation that had been recorded in 1918, due to pneumonic flu. While the pace of population growth has slowed down, and
immigration flows remain its important component, the population is ageing.
The significant fall in the number of marriages, the increasing of average age at marriage, and the rise in both the number of wedlock births and the number of divorces, influence the new familiar models in Portugal.
This paper intends to study the patterns of demographic variables responsible for the major changes in both population size and age structure of the population and culminating in its significant ageing.

Key words: population, fertility, mortality, expectancy of life, migrations, ageing.