Primary elections in the United States have undergone enormous change in recent decades and reformers have continued to push for further changes. In this dissertation I seek to investigate how the differences among these primary elections affect both candidates and voters, with a particular focus on the top-two primary, which has been largely excluded from the previous literature. I evaluate the effect of primary elections systems on candidate ideology, electoral competition, and voter participation to determine what differences, if any, exist among these primary election systems. I find that significant differences in candidate ideology, electoral competition, and voter participation do exist among these primary election systems.