One in six nonelderly Americans lives in families in which the adults work at least half-time on average but whose incomes fall below twice the federal poverty level. The primary earner in working low-income families typically works full-time, year-round. While the work effort of the primary earner in low-income working families matches that of higher-income families, other adults in these families work less. Even if all able-bodied adults ages 25-54 in these families worked full-time, we find that only about 20 percent of these families would increase their earnings to above 200 percent of the federal poverty line.