 Well, the most obvious and most certain challenge is the fiscal challenge. Graying means paying. It means paying more for pensions, more for health care, more for social services for the elderly. Most of the developed countries have paves-you-go social insurance systems in which the current working age population is taxed to pay for benefits to the current retired population. Paves-you-go systems are vulnerable to demographic aging. The falling fertility and rising longevity means a declining ratio of workers to retirees and a rising cost rate for paves-you-go social insurance systems. As part of our aging vulnerability project at CSIS, we project that most developed countries will see an extra 10 percent, we'll have to devote an extra 10 percent of GDP to benefits to the elderly over the next 25 to 30 years. So that would bring the total. It varies from country to country, but from an average of about 12 percent to an average of about 24 percent. That's a big increase. An extra 10 percent of GDP is 25 percent of workers' wages. It's also three times what the United States now spends as a share of GDP, or two and a half times what we now spend on national defense. The first line challenge is the fiscal challenge, but the implications reach far beyond that. There's also the challenge of more slowly growing or declining working age populations, which in turn will mean, in all likelihood, slower economic growth. Falling fertility is not only hollowing out the traditional age pyramid at the bottom, it's also ushering in an era of widespread workforce and population decline. In many European countries and in Japan, the workforce will be declining by between one and one and a half percent per year by the 2020s. And this will translate into slower aggregate GDP growth and perhaps also into slower per capita GDP growth. More slowly growing workforces mean relatively less investment and perhaps fewer opportunities for innovation and technological advance, what some economists call learning by doing. The developed countries will have an aging stock of physical capital. They'll also have an aging stock of human capital as the median age of the workforce rises. There's also a social and a political challenge. Will smaller families be able to care for a burgeoning number of frail elders? Although most countries have socialized much of the cost in the developed world of growing old with pension, through pension programs and acute care, health benefits, health benefits, the cost of long-term care in many places. Certainly this is true in the United States. Still largely falls on the extended family. Will the family be able to bear this added burden or will the cost get pushed off onto government with even more dire implications for the long-term fiscal outlook? Will aging electorates become, will the gray lobby become a political titan astride the body politic? What does the aging of the electorate imply for the ability to reform unsustainable old age benefit programs in the future? And most broadly, what about the culture? What about the social mood? Sociology, sociologists have spent much time talking about the individual life cycle and trying to isolate attitudes and behaviors typical to different ages, risk-taking entrepreneurial youth and more risk-averse, cautious old age. Cicero said, young men for action, old men for counsel. Is there an analogy to the society as a whole as the median age rises and what does that imply for the dynamism of the culture and of society in the future? The well-known French demographer, the late French demographer, Alfred Sovi talked about the future of Europe as a future of old people living in old houses ruminating about old ideas. And finally, the geopolitical challenge. The developed world is aging and many parts of it will be shrinking, but much of the developing world will still be young and fast growing. What is this, what are differential rates of population growth and differential age structures imply for the geopolitical balance of power in the future? It's not just a question of manpower shortages or people power shortages. It's also a question of fiscal pressure on defense and foreign affairs budgets and to loop back around to the question of social mood. Will older electorates be less prone to engage issues in the broader geopolitical theater or will they be more prone to protect current consumption? So a whole range of challenges, some of them relatively certain, some more speculative, but all of which need to be considered by policymakers.