Who owns what? How is the ownership of wealth distributed among families? Both basic research and practical concerns demand information about how many families are among the top wealth holders, how many families are truly asset-poor, and how many fall somewhere in the middle. Understanding wealth inequality also demands that we know how much these families own and how both levels of ownership and the extent of inequality have changed over time. What did the distribution of wealth look like in past decades, and what does it look like now? Few would expect wealth to be distributed evenly. Life-cycle patterns, baby booms, and other demographic trends affect the number of people earning income and accumulating assets at any time. Individual preferences for owning certain types of assets and debts vary over time as well as across generations at single points in time. Structural constraints limit the ability of entire groups of people to do the things necessary to accumulate assets and may even make them more likely to own debt. The availability of financial instruments varies over time, and laws regarding saving, investment, and related taxes affect the propensity of households to save. Of course, luck also plays an important role in distributing wealth unevenly.