The chapter examines the housing affordability problem in the U.S. through the lens of the shelter poverty concept. "Shelter poverty" challenges the conventional view that a household can reasonably afford up to a certain percent of income -- currently thirty percent -- for housing without hardship. It offers instead a sliding scale that takes into account differences in household composition and income in determining how much reasonably can be afforded for housing without compromising non-shelter necessities. Following a discussion of conceptual and methodological issues around housing affordability, the chapter summarizes the contours of housing affordability in the U.S. at the beginning of the 21st century, as well as trend since the 1970s.