With the enrollment of students critical to our society economy, and the mission and financial sustainability of most higher education institutions, understanding the relationship between population changes and enrollment is important. A group of institutions that may be the most vulnerable to population changes are four-year regional public institutions. Regional public colleges and universities tend to receive inadequate state support compared to other public peers and have limited options for increasing net tuition revenue. As a result, reductions in the population of potential students could have severe impacts on regional publics and their ability to offer necessary programs or remain fiscally sustainable. This dissertation examines the relationship between cohort changes of full-time equivalent high school graduating cohort (18 year olds) and the outcome variables of institutional undergraduate full-time enrollment and net-tuition revenue. This dissertation first studies non-profit Title IV-eligible institutions of higher education and then focuses specifically on regional public colleges and universities. To study my research questions, I use a panel fixed effects model that controls for state-level time-invariant factors and national trends over time. To determine if using different classifications for regional public institutions changes the results, I use various classifications of regional publics reported in the literature. The results indicate that during the full study period of 2000-2018, there were differences in the enrollment and net tuition revenue responses to cohort changes by public and private institutional types. For public institutions, there was a moderate to strong relationship between the cohort size of 18 year old individuals and both undergraduate full-time equivalent enrollment and net tuition revenue, especially for regional public colleges and universities. For private institutions, the relationship between cohort size and enrollment and net tuition revenue was moderate, with research privates having almost no relationship and baccalaureate privates having a moderate to strong relationship. When using the same estimation for different time periods relating to the great recession (2000-2007, 2008-2010, 2011-2018), the effects on enrollment were similar across time periods and the effects on net tuition revenue were slightly stronger during the 2011-2018 period. Overall, the results indicate that cohort changes matter for all classifications of regional publics. The findings extend upon previous research by exploring a new period of the 2000s and 2010s, two decades characterized as periods of momentous change for higher education. The findings also explore enrollment and net tuition elasticities for new classifications of regional public institutions. Finally, this dissertation's findings help inform future academic, budget, and policy planning decisions at the institution, state, and national levels.