This paper considers international migration in the context of population ageing. In many Western countries, the search for appropriate responses to manage future population ageing and population decline has directed attention to international migration. It seems reasonable to believe that international migrants, mostly of young working age, can supply population deficits created by low birth rates, protect European society and economy from the economic costs of elderly dependency, and provide a workforce to care for the elderly. Particular prominence has been given to this option through the publicity attendant on a report from the UN Population Division in 2000 on 'replacement migration', which has been widely reported and widely misunderstood. Although immigration can prevent population decline, it is already well known that it can only prevent population ageing at unprecedented, unsustainable and increasing levels of inflow, which would generate rapid population growth and eventually displace the original population from its majority position. This paper reviews these arguments in the context of the causes and inevitability of population ageing, with examples mostly based on UK data. It discusses various options available in response to population ageing through workforce, productivity, pensions reform and other means. It concludes that there can be no 'solution' to population ageing, which is to a considerable degree unavoidable. However, if the demographic regime of the United Kingdom continues to be relatively benign, future population ageing can be managed with tolerable inconvenience without recourse to increased immigration for 'demographic' purposes. At present (2001), net immigration to the United Kingdom is already running at record levels and is now the main engine behind UK population and household growth. By itself, population stabilization, or even mild reduction, is probably to be welcomed in the United Kingdom, although the issue has attracted little attention since the 1970s.