This thesis investigates individual differences and their impact on reading proficiency using different measures of proficiency, a variety of data collection and statistical methods, and different populations. The goal was to examine the impact that individual differences in certain reading-related skills and cognitive abilities have on reading proficiency and how these differences interact. Through three key studies that make up this thesis, several important discoveries and contributions were made to the field. Chapter 2 introduces an easy-to-use application for measuring cloze probability. ‘ClozApp’, was created and made publicly available, along with a user manual and sample code for programming. Chapter 3 contributed through the development of a novel statistical method used to analyze variance between populations with different linguistic backgrounds. This method was used to demonstrate how an individual’s linguistic background (i.e., whether they were first- or second-language speakers of English) impacted how individual differences in reading skills influence their reading fluency, as indicated through their eye-movements. This statistical prediction method is open source and was made widely available for use along with sample data and code. In Chapter 4, a new connection was found between two important cognitive factors that are well-known in the reading literature: statistical learning and motivation. Using mediation analyses, this project discovered an interaction between these factors that further highlights the ways they impact reading proficiency. This thesis demonstrates a comprehensive approach to investigating individual differences in reading proficiency in the following ways: (i) both reading fluency and comprehension were investigated as measures of reading proficiency, (ii) data collection included a variety of reading-related skills, cognitive abilities, and group differences, and (iii) unique statistical analysis methods were utilized to investigate both individual and group differences. This thesis highlights important new discoveries and makes significant lasting contributions to the field of reading research. / Thesis / Doctor of Philosophy (PhD) / This thesis investigates how individual differences influence reading proficiency. Specifically, it asks how the ways in which people differ on certain reading-related skills and cognitive abilities can determine how well they read. Using different measures of proficiency, a variety of data collection and statistical methods, and looking across different populations, the goal of this thesis was to examine the ways in which people differ in these skills and abilities, how these differences interact, and the resulting impact on reading proficiency. This thesis resulted in three significant contributions to the field. First, it made available a new application for collecting data on an important variable in reading research – cloze probability. In addition, it culminated in the development of a novel statistical method that demonstrates how an individual’s linguistic background can influence their reading fluency. Finally, a new connection was found between two important cognitive factors that interact to influence reading comprehension.