The Portuguese population is becoming older and its present size is the result of accelerated increase occurring over a short period of time. Migrations are currently the main factor affecting changes in developed countries, due to the substantial reduction in natural balances, and Portugal is no exception. The aim is to study the characteristics of the demographic variables responsible for major changes occurring in the age structure of the population and culminating in its significant ageing. Since the ageing phenomenon is a lengthy process, the analysis applies for the most part to a period of roughly forty years.

keywords: fertililty, ageing, foreign population.