How much will the United States spend on health care during the next decade or two? The answer matters greatly to physicians, federal and state governments, businesses, and the general public. The answer will determine the type and extent of care that physicians can provide to their patients, as well as the amount of physicians' take-home pay. It will also determine how much everyone else can consume or invest in other goods and services. Unfortunately, forecasting health care spending is extremely difficult. Future spending depends in part on developments within the health care sector and in part on developments in the economy as a whole. The former include changes in the prevalence of health problems such as obesity, infectious diseases, and dementia, as well as changes in medical technology such as new drugs, imaging devices, and surgical procedures. The economy as a whole includes variables such as the unemployment rate, trends in average wages, and prices of securities and housing.