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Time to completion of the baccalaureate degree has increased markedly among college graduates in the United States over the last three decades. Between the cohorts graduating from high school in 1972 and 1992, average time to degree increased by more than one-quarter of a year, the completion rate among college attendees dropped from 51.1% to 45.3% and, among those receiving degrees, the percent receiving a degree within 4 years dropped from 56.8% to 43.6%. Using data from the NLS72 and NELS:88 longitudinal surveys, we assess the extent to which these shifts result from changes in the preparation of college students over time, reductions in collegiate resources, an erosion in family circumstances, or other broad macro-economic adjustments. We show that the increase in time to degree is localized among those who begin postsecondary education at public colleges outside the most selective flagship universities. The net effect of changes in student characteristics including pre-collegiate achievement and parental characteristics does not explain the observed increase in time to degree or the drop in completion rates. We produce evidence that increased stratification in U.S. higher education and reductions in collegiate resources outside the top-tier of institutions are a primary component of the explanation for the observed increases in time to degree, with increases in time to degree relatively concentrated in states that have experienced rapid growth in the size of the college-age population and dilution in resources per student at many public colleges. The shift toward initial enrollment at two-year institutions rather than four-year institutions accounts for some of the decline in completion rates. In addition, we find evidence of increased hours of employment among students, which is consistent with students working more to meet rising college costs.