This report uses data from the National Postsecondary Student Aid Study and the Beginning Postsecondary Students Longitudinal Study to examine how low income undergraduates (with family incomes below 125 percent of the federally established poverty threshold) finance their postsecondary education, focusing on the importance of financial aid. The report uses narratives, tables, and graphs to describe the demographic characteristics and enrollment patterns of low income students and compares them with other undergraduates. It examines dependent, single independent, and independent students with dependents separately. It examines students' financial needs, the type of financial aid they receive and the amount and the relationship between financial aid and cost. Next, it describes two important sources of support other than financial aid: parental support and work. Finally, the report examines persistence and attainment among low income students who enrolled in postsecondary education in 1989-90. A concluding section notes that financial aid, parental contributions, and work do not add up to estimated costs and possible reasons are suggested. Among highlights are: that 20 percent of all undergraduates are from low-income families; that 88 percent of these students received financial aid averaging $5,800; and that there was an average unmet need of $4,900. A glossary and methodological information are appended. (Contains 20 tables and 11 figures.) (DB)