The first part of the current century, in Portugal, is marked by the low fertility, the increase of life expectancy and a sharp decline in immigration. The pace of population growth continues to slowing down and the immigration flows
remain its most important component. The substantial fall in the number of marriages, the rise in both the number of wedlock births and the average age at marriage and the increasing tendency in 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.