The stuff that enables people to earn a living. Human capital can be increased by investing in education, training and health care. Economists increasingly argue that the accumulation of human as well as physical capital (plant and machinery) is a crucial ingredient of economic growth, par¬ticularly in the new economy. Even so, this conclusion is largely a matter of theory and faith, rather than the result of detailed empirical analysis. Economists have made little progress in solving the tricky problem of how to measure human capital, even within the same country over time, let alone for comparisons between countries. Levels of spending on, say, education are not necessarily a good indicator of how much human capital an education system is creating; indeed, some economists argue that higher education spending may be a consequence of a country becoming wealthy rather than a cause. Never the less, even modest estimates of the stock of human capital in most countries suggests that it would pay to greatly increase investment in medical technologies that would extend the working lives of most people. The non-economic benefits would be worth having, too.