This article identifies the factors that change the stock of market housing affordable to low-income households. During the past 15 years policy makers and academics have concentrated on the shortage of good-quality affordable housing for low-income households. Among the triggers of that interest are the increase in homelessness, the disappearance of rental units affordable to the least well off members of society, and the implications of growing income inequality. The authors take advantage of the panel nature of the metropolitan surveys of the American Housing Survey to model the movement of individual housing units in and out of the stock of units affordable to low-income households. A multinomial logit methodology is used to estimate the effects of unit, neighborhood, and market characteristics and conditions on the status of a unit over time. The authors compare the alternative outcomes for an affordable unit with the outcomes for unaffordable rental stock. One objective of this comparison is to determine whether these factors have symmetric effects across different segments of the housing market. The empirical results suggest that movements are more sensitive to variation in neighborhood conditions than to unit characteristics or movements in market rents or prices.