In this essay, we review the basic economics of housing supply and the functioning of U.S. housing markets to better understand the impacts on home prices, household wealth and the spatial distribution of people across markets. Section II documents the state of housing affordability in the U.S., and begins with three core facts about housing supply. First, when building is unrestricted by regulation or geography, housing supply curves seem relatively flat, meaning that we can approximate reality by referring to a single production cost. Second, both geography and regulation severely restrict the ease of building in some parts of the country. These constraints raise building costs both directly, by increasing time delays and reducing the amount of available land, and indirectly, by ensuring the homes are produced more on a one-by-one basis rather than in bulk. Third, the supply of housing is kinked and vertical downwards because housing is durable.